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/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 {};
 }