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/child_window.rs

@@ -13,6 +13,7 @@ fn main() -> Result<(), impl std::error::Error> {
     #[path = "util/fill.rs"]
     mod fill;
 
+    #[derive(Debug)]
     struct WindowData {
         window: Box<dyn Window>,
         color: u32,
@@ -24,7 +25,7 @@ fn main() -> Result<(), impl std::error::Error> {
         }
     }
 
-    #[derive(Default)]
+    #[derive(Default, Debug)]
     struct Application {
         parent_window_id: Option<WindowId>,
         windows: HashMap<WindowId, WindowData>,

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,

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>>,
 }

examples/pump_events.rs

@@ -16,7 +16,7 @@ fn main() -> std::process::ExitCode {
     #[path = "util/fill.rs"]
     mod fill;
 
-    #[derive(Default)]
+    #[derive(Default, Debug)]
     struct PumpDemo {
         window: Option<Box<dyn Window>>,
     }

examples/run_on_demand.rs

@@ -14,7 +14,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     #[path = "util/fill.rs"]
     mod fill;
 
-    #[derive(Default)]
+    #[derive(Default, Debug)]
     struct App {
         idx: usize,
         window_id: Option<WindowId>,

examples/window.rs

@@ -12,7 +12,7 @@ use winit::window::{Window, WindowAttributes, WindowId};
 #[path = "util/fill.rs"]
 mod fill;
 
-#[derive(Default)]
+#[derive(Default, Debug)]
 struct App {
     window: Option<Box<dyn Window>>,
 }

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>>,