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/control_flow.rs

@@ -46,7 +46,7 @@ fn main() -> Result<(), impl std::error::Error> {
     event_loop.run_app(ControlFlowDemo::default())
 }
 
-#[derive(Default)]
+#[derive(Default, Debug)]
 struct ControlFlowDemo {
     mode: Mode,
     request_redraw: bool,