Properly implement Debug for Window and EventLoop types (#3297)

For EventLoop, EventLoopBuilder, EventLoopProxy and by requiring it as
a supertrait of Window and ActiveEventLoop.

It is especially useful for user to be able to know that Window is Debug.

src/platform_impl/apple/uikit/event_loop.rs

@@ -116,6 +116,7 @@ impl HasDisplayHandle for OwnedDisplayHandle {
     }
 }
 
+#[derive(Debug)]
 pub struct EventLoop {
     mtm: MainThreadMarker,
     window_target: ActiveEventLoop,

src/platform_impl/apple/uikit/window.rs

@@ -459,6 +459,7 @@ impl Inner {
     }
 }
 
+#[derive(Debug)]
 pub struct Window {
     inner: MainThreadBound<Inner>,
 }