iOS: Refactor event handling to share code with macOS (#3865)

Instead of storing the event handler within the AppState, and extracting
it our every time we need it, we now use the same event handling
implementation as for macOS that ensures we don't re-entrantly call the
event handler, and that we un-register the handler again after we're
done using it (`UIApplicationMain` won't return, but may still unwind,
so this is very important for panic safety).

src/platform_impl/apple/appkit/mod.rs

@@ -5,7 +5,6 @@ mod app;
 mod app_state;
 mod cursor;
 mod event;
-mod event_handler;
 mod event_loop;
 mod ffi;
 mod menu;