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/linux/x11/dnd.rs

@@ -39,6 +39,7 @@ impl From<io::Error> for DndDataParseError {
     }
 }
 
+#[derive(Debug)]
 pub struct Dnd {
     xconn: Arc<XConnection>,
     // Populated by XdndEnter event handler