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/mod.rs

@@ -231,6 +231,7 @@ unsafe extern "C" fn x_error_callback(
     0
 }
 
+#[derive(Debug)]
 pub enum EventLoop {
     #[cfg(wayland_platform)]
     Wayland(Box<wayland::EventLoop>),