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/x11_embed.rs

@@ -12,6 +12,7 @@ fn main() -> Result<(), Box<dyn Error>> {
     #[path = "util/fill.rs"]
     mod fill;
 
+    #[derive(Debug)]
     pub struct XEmbedDemo {
         parent_window_id: u32,
         window: Option<Box<dyn Window>>,