api: add ApplicationHandler and matching run APIs

Add a simple `ApplicationHandler` trait since winit is moving towards
trait based API. Add `run_app` group of APIs to accept `&mut impl
ApplicationHandler` deprecating the old `run` APIs.

Part-of: https://github.com/rust-windowing/winit/issues/3432

src/platform_impl/ios/event_loop.rs

@@ -182,7 +182,7 @@ impl<T: 'static> EventLoop<T> {
             application.is_none(),
             "\
                 `EventLoop` cannot be `run` after a call to `UIApplicationMain` on iOS\n\
-                 Note: `EventLoop::run` calls `UIApplicationMain` on iOS",
+                 Note: `EventLoop::run_app` calls `UIApplicationMain` on iOS",
         );
 
         let handler = map_user_event(handler, self.receiver);