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/web/window.rs

@@ -1,4 +1,5 @@
 use std::cell::Ref;
+use std::fmt;
 use std::rc::Rc;
 use std::sync::Arc;
 
@@ -23,6 +24,12 @@ pub struct Window {
     inner: Dispatcher<Inner>,
 }
 
+impl fmt::Debug for Window {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Window").finish_non_exhaustive()
+    }
+}
+
 pub struct Inner {
     id: WindowId,
     pub window: web_sys::Window,