Replace remaining AppKit bindings with icrate's (#3296)

* Use icrate's window structs and enums

* Properly implement protocols

* Use icrate's NSWindow

We were previously using undocumented methods on `NSWindowTabGroup`

* Use icrate's NSApplication

And clean up some doc comments regarding NSApplication

src/platform_impl/macos/mod.rs

@@ -4,7 +4,6 @@ mod util;
 mod app;
 mod app_delegate;
 mod app_state;
-mod appkit;
 mod cursor;
 mod event;
 mod event_loop;