macOS: Refactor event handler storage (#3532)

This makes our use of `unsafe` to make the event handler temporarily 'static be local to a module, in a way that's (hopefully) much easier to reason about.

src/platform_impl/macos/mod.rs

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