LibGUI+Everywhere: Use fallible Window::set_main_widget() everywhere :^)

Rip that bandaid off!

This does the following, in one big, awkward jump:
- Replace all uses of `set_main_widget<Foo>()` with the `try` version.
- Remove `set_main_widget<Foo>()`.
- Rename the `try` version to just be `set_main_widget` because it's now
  the only one.

The majority of places that call `set_main_widget<Foo>()` are inside
constructors, so this unfortunately gives us a big batch of new
`release_value_but_fixme_should_propagate_errors()` calls.

Userland/DevTools/GMLPlayground/main.cpp

@@ -84,14 +84,14 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     window->set_icon(app_icon.bitmap_for_size(16));
     window->resize(800, 600);
 
-    auto splitter = TRY(window->try_set_main_widget<GUI::HorizontalSplitter>());
+    auto splitter = TRY(window->set_main_widget<GUI::HorizontalSplitter>());
     auto editor = TRY(splitter->try_add<GUI::TextEditor>());
     auto preview_frame_widget = TRY(splitter->try_add<GUI::Frame>());
 
     auto preview_window = TRY(GUI::Window::try_create());
     preview_window->set_title("Preview - GML Playground");
     preview_window->set_icon(app_icon.bitmap_for_size(16));
-    auto preview_window_widget = TRY(preview_window->try_set_main_widget<GUI::Widget>());
+    auto preview_window_widget = TRY(preview_window->set_main_widget<GUI::Widget>());
     preview_window_widget->set_fill_with_background_color(true);
 
     GUI::Widget* preview = preview_frame_widget;

Userland/DevTools/HackStudio/Dialogs/Git/GitCommitDialog.cpp

@@ -17,13 +17,13 @@ GitCommitDialog::GitCommitDialog(GUI::Window* parent)
     set_title("Commit");
     set_icon(parent->icon());
 
-    auto& widget = set_main_widget<GUI::Widget>();
-    widget.load_from_gml(git_commit_dialog_gml);
+    auto widget = set_main_widget<GUI::Widget>().release_value_but_fixme_should_propagate_errors();
+    widget->load_from_gml(git_commit_dialog_gml);
 
-    m_message_editor = widget.find_descendant_of_type_named<GUI::TextEditor>("message_editor");
-    m_cancel_button = widget.find_descendant_of_type_named<GUI::Button>("cancel_button");
-    m_commit_button = widget.find_descendant_of_type_named<GUI::Button>("commit_button");
-    m_line_and_col_label = widget.find_descendant_of_type_named<GUI::Label>("line_and_col_label");
+    m_message_editor = widget->find_descendant_of_type_named<GUI::TextEditor>("message_editor");
+    m_cancel_button = widget->find_descendant_of_type_named<GUI::Button>("cancel_button");
+    m_commit_button = widget->find_descendant_of_type_named<GUI::Button>("commit_button");
+    m_line_and_col_label = widget->find_descendant_of_type_named<GUI::Label>("line_and_col_label");
 
     m_message_editor->on_change = [this]() {
         m_commit_button->set_enabled(!m_message_editor->text().is_empty() && on_commit);

Userland/DevTools/HackStudio/Dialogs/NewProjectDialog.cpp

@@ -49,10 +49,10 @@ NewProjectDialog::NewProjectDialog(GUI::Window* parent)
     set_resizable(false);
     set_title("New project");
 
-    auto& main_widget = set_main_widget<GUI::Widget>();
-    main_widget.load_from_gml(new_project_dialog_gml);
+    auto main_widget = set_main_widget<GUI::Widget>().release_value_but_fixme_should_propagate_errors();
+    main_widget->load_from_gml(new_project_dialog_gml);
 
-    m_icon_view_container = *main_widget.find_descendant_of_type_named<GUI::Widget>("icon_view_container");
+    m_icon_view_container = *main_widget->find_descendant_of_type_named<GUI::Widget>("icon_view_container");
     m_icon_view = m_icon_view_container->add<GUI::IconView>();
     m_icon_view->set_always_wrap_item_labels(true);
     m_icon_view->set_model(m_model);
@@ -65,24 +65,24 @@ NewProjectDialog::NewProjectDialog(GUI::Window* parent)
             do_create_project();
     };
 
-    m_description_label = *main_widget.find_descendant_of_type_named<GUI::Label>("description_label");
-    m_name_input = *main_widget.find_descendant_of_type_named<GUI::TextBox>("name_input");
+    m_description_label = *main_widget->find_descendant_of_type_named<GUI::Label>("description_label");
+    m_name_input = *main_widget->find_descendant_of_type_named<GUI::TextBox>("name_input");
     m_name_input->on_change = [&]() {
         update_dialog();
     };
-    m_create_in_input = *main_widget.find_descendant_of_type_named<GUI::TextBox>("create_in_input");
+    m_create_in_input = *main_widget->find_descendant_of_type_named<GUI::TextBox>("create_in_input");
     m_create_in_input->on_change = [&]() {
         update_dialog();
     };
-    m_full_path_label = *main_widget.find_descendant_of_type_named<GUI::Label>("full_path_label");
+    m_full_path_label = *main_widget->find_descendant_of_type_named<GUI::Label>("full_path_label");
 
-    m_ok_button = *main_widget.find_descendant_of_type_named<GUI::Button>("ok_button");
+    m_ok_button = *main_widget->find_descendant_of_type_named<GUI::Button>("ok_button");
     m_ok_button->set_default(true);
     m_ok_button->on_click = [this](auto) {
         do_create_project();
     };
 
-    m_cancel_button = *main_widget.find_descendant_of_type_named<GUI::Button>("cancel_button");
+    m_cancel_button = *main_widget->find_descendant_of_type_named<GUI::Button>("cancel_button");
     m_cancel_button->on_click = [this](auto) {
         done(ExecResult::Cancel);
     };

Userland/DevTools/HackStudio/Editor.cpp

@@ -94,7 +94,7 @@ ErrorOr<void> Editor::initialize_tooltip_window()
         s_tooltip_window->set_window_type(GUI::WindowType::Tooltip);
     }
     if (s_tooltip_page_view.is_null()) {
-        s_tooltip_page_view = TRY(s_tooltip_window->try_set_main_widget<WebView::OutOfProcessWebView>());
+        s_tooltip_page_view = TRY(s_tooltip_window->set_main_widget<WebView::OutOfProcessWebView>());
     }
     return {};
 }

Userland/DevTools/HackStudio/Locator.cpp

@@ -149,7 +149,7 @@ Locator::Locator(Core::Object* parent)
     m_popup_window->set_window_type(GUI::WindowType::Popup);
     m_popup_window->set_rect(0, 0, 500, 200);
 
-    m_suggestion_view = m_popup_window->set_main_widget<GUI::TableView>();
+    m_suggestion_view = m_popup_window->set_main_widget<GUI::TableView>().release_value_but_fixme_should_propagate_errors();
     m_suggestion_view->set_column_headers_visible(false);
 
     m_suggestion_view->on_activation = [this](auto& index) {

Userland/DevTools/Inspector/main.cpp

@@ -99,7 +99,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     }));
     help_menu.add_action(GUI::CommonActions::make_about_action("Inspector", app_icon, window));
 
-    auto widget = TRY(window->try_set_main_widget<GUI::Widget>());
+    auto widget = TRY(window->set_main_widget<GUI::Widget>());
     widget->set_fill_with_background_color(true);
     widget->set_layout<GUI::VerticalBoxLayout>();
 

Userland/DevTools/Profiler/main.cpp

@@ -86,7 +86,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     window->set_icon(app_icon.bitmap_for_size(16));
     window->resize(800, 600);
 
-    auto main_widget = TRY(window->try_set_main_widget<GUI::Widget>());
+    auto main_widget = TRY(window->set_main_widget<GUI::Widget>());
     main_widget->set_fill_with_background_color(true);
     main_widget->set_layout<GUI::VerticalBoxLayout>();
 
@@ -318,19 +318,19 @@ static bool prompt_to_stop_profiling(pid_t pid, DeprecatedString const& process_
     window->set_icon(Gfx::Bitmap::try_load_from_file("/res/icons/16x16/app-profiler.png"sv).release_value_but_fixme_should_propagate_errors());
     window->center_on_screen();
 
-    auto& widget = window->set_main_widget<GUI::Widget>();
-    widget.set_fill_with_background_color(true);
-    auto& layout = widget.set_layout<GUI::VerticalBoxLayout>();
+    auto widget = window->set_main_widget<GUI::Widget>().release_value_but_fixme_should_propagate_errors();
+    widget->set_fill_with_background_color(true);
+    auto& layout = widget->set_layout<GUI::VerticalBoxLayout>();
     layout.set_margins({ 0, 0, 16 });
 
-    auto& timer_label = widget.add<GUI::Label>("...");
+    auto& timer_label = widget->add<GUI::Label>("...");
     Core::ElapsedTimer clock;
     clock.start();
     auto update_timer = Core::Timer::construct(100, [&] {
         timer_label.set_text(DeprecatedString::formatted("{:.1} seconds", clock.elapsed() / 1000.0f));
     });
 
-    auto& stop_button = widget.add<GUI::Button>("Stop");
+    auto& stop_button = widget->add<GUI::Button>("Stop");
     stop_button.set_fixed_size(140, 22);
     stop_button.on_click = [&](auto) {
         GUI::Application::the()->quit();

Userland/DevTools/SQLStudio/main.cpp

@@ -30,7 +30,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     window->set_icon(app_icon.bitmap_for_size(16));
     window->set_title("SQL Studio");
 
-    auto main_widget = TRY(window->try_set_main_widget<MainWidget>());
+    auto main_widget = TRY(window->set_main_widget<MainWidget>());
     main_widget->initialize_menu(window);
 
     window->on_close_request = [&] {