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.

examples/dnd.rs

@@ -20,6 +20,7 @@ fn main() -> Result<(), Box<dyn Error>> {
 }
 
 /// Application state and event handling.
+#[derive(Debug)]
 struct Application {
     window: Option<Box<dyn Window>>,
 }