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/Applets/WorkspacePicker/DesktopStatusWindow.cpp

@@ -148,7 +148,7 @@ DesktopStatusWindow::DesktopStatusWindow()
 {
     set_window_type(GUI::WindowType::Applet);
     set_has_alpha_channel(true);
-    m_widget = &set_main_widget<DesktopStatusWidget>();
+    m_widget = set_main_widget<DesktopStatusWidget>().release_value_but_fixme_should_propagate_errors();
 }
 
 void DesktopStatusWindow::wm_event(GUI::WMEvent& event)