macOS: Merge window and delegate state (#3391)

Previously we had a sort of artificial split between these, but both were accessing each other's state, since it's really the same state!
It was especially difficult to follow what happens to the fullscreen state.
So instead, we basically merge the window and the delegate files.

This does unfortunately screw a bit with the git history, apologies to whoever reads this in the future!

src/platform_impl/macos/event_loop.rs

@@ -75,7 +75,7 @@ impl PanicInfo {
 #[derive(Debug)]
 pub struct EventLoopWindowTarget {
     delegate: Id<ApplicationDelegate>,
-    mtm: MainThreadMarker,
+    pub(super) mtm: MainThreadMarker,
 }
 
 impl EventLoopWindowTarget {