Make EventLoopProxy Sync

Co-authored-by: daxpedda <daxpedda@gmail.com>
Closes: #3448

tests/sync_object.rs

@@ -1,6 +1,15 @@
 #[allow(dead_code)]
 fn needs_sync<T: Sync>() {}
 
+#[test]
+fn event_loop_proxy_send() {
+    #[allow(dead_code)]
+    fn is_send<T: 'static + Send>() {
+        // ensures that `winit::EventLoopProxy<T: Send>` implements `Sync`
+        needs_sync::<winit::event_loop::EventLoopProxy<T>>();
+    }
+}
+
 #[test]
 fn window_sync() {
     // ensures that `winit::Window` implements `Sync`