Drop application handler on run loop exit (#4149)

Calling the `Drop` impl of the user's `ApplicationHandler` is important on
iOS and Web, since they don't return from `EventLoop::run_app`.

And now that we reliably call `Drop`, the `ApplicationHandler::exited`
event/callback is unnecessary; using `Drop` composes much better (open files
etc. stored in the app state will be automatically flushed), and prevents
weirdness like attempting to create a new window while exiting.

src/changelog/unreleased.md

@@ -228,6 +228,8 @@ changelog entry.
 - Remove `Window::inner_position`, use the new `Window::surface_position` instead.
 - Remove `CustomCursorExtWeb`, use the `CustomCursorSource`.
 - Remove `CustomCursor::from_rgba`, use `CustomCursorSource` instead.
+- Removed `ApplicationHandler::exited`, the event loop being shut down can now be listened to in
+  the `Drop` impl on the application handler.
 
 ### Fixed