macOS/iOS: Various refactorings in application state (#3720)

I'm preparing to get rid of our application delegate in favour of registering
notification observers, to do so I'm renaming `app_delegate.rs` to
`app_state.rs`, and moving the functionality out of the Objective-C method
into a normal method.

Additionally, `AppState` previously implemented `Default`, but really, this
was a hack done because someone (probably myself) was too lazy to write out
the full initialization in `AppDelegate::new`.

src/platform_impl/macos/mod.rs

@@ -2,7 +2,7 @@
 mod util;
 
 mod app;
-mod app_delegate;
+mod app_state;
 mod cursor;
 mod event;
 mod event_handler;