Drop application handler on run loop exit (#4149)

Calling the `Drop` impl of the user's `ApplicationHandler` is important on
iOS and Web, since they don't return from `EventLoop::run_app`.

And now that we reliably call `Drop`, the `ApplicationHandler::exited`
event/callback is unnecessary; using `Drop` composes much better (open files
etc. stored in the app state will be automatically flushed), and prevents
weirdness like attempting to create a new window while exiting.

examples/application.rs

@@ -592,18 +592,18 @@ impl ApplicationHandler for Application {
         }
     }
 
-    #[cfg(not(android_platform))]
-    fn exiting(&mut self, _event_loop: &dyn ActiveEventLoop) {
-        // We must drop the context here.
-        self.context = None;
-    }
-
     #[cfg(target_os = "macos")]
     fn macos_handler(&mut self) -> Option<&mut dyn ApplicationHandlerExtMacOS> {
         Some(self)
     }
 }
 
+impl Drop for Application {
+    fn drop(&mut self) {
+        info!("Application exited");
+    }
+}
+
 #[cfg(target_os = "macos")]
 impl ApplicationHandlerExtMacOS for Application {
     fn standard_key_binding(