Event for when the application will close

[DJMcNab]

We have:

glazier/src/window.rs

Lines 773 to 779 in 618382a

	/// Called when the shell requests to close the window, for example because the user clicked
	/// the little "X" in the titlebar.
	///
	/// If you want to actually close the window in response to this request, call
	/// [`WindowHandle::close`]. If you don't implement this method, clicking the titlebar "X" will
	/// have no effect.
	fn request_close(&mut self) {}

However, we don't report when the actual application is closing. This is most useful on macOS, as in other desktop environments (generally) the application is finished when the final window is closed¹.
But for consistency, we should have the same API across all backends. This could be used for saving states, for example.
This would be slightly earlier than AppHandler::drop - the event loop might still be running at this point (?)

An implementation for macOS exists in 86f0494 (from lapce's fork)

An implementation note:
This should not be called upon calling Application::quit, for consistency with the Window methods.

AppHandler::drop could possibly be used in that case?

CC @waywardmonkeys, as discussed in Zulip.

Footnotes

1. Customising this behaviour could make sense - e.g. for close to tray on Windows (and Linux in theory, except that there's no tray) ↩