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`.
Changes relevant to Winit:
- `icrate` has been deprecated in favour of separate crates per
framework, in our case `objc2-foundation` and `objc2-app-kit` (and in
the future `objc2-ui-kit` on iOS).
- Moved `MainThreadMarker::run_on_main` to free-standing function
`run_on_main`.
- Changed how features work, this should result in less code that we
need to compile.
- Enums are now real structs instead of type-aliases and free constants.
