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/apple/appkit/event_loop.rs

@@ -1,5 +1,6 @@
 use std::any::Any;
 use std::cell::Cell;
+use std::fmt;
 use std::panic::{catch_unwind, resume_unwind, RefUnwindSafe, UnwindSafe};
 use std::rc::{Rc, Weak};
 use std::sync::Arc;
@@ -39,6 +40,12 @@ pub struct PanicInfo {
     inner: Cell<Option<Box<dyn Any + Send + 'static>>>,
 }
 
+impl fmt::Debug for PanicInfo {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("PanicInfo").finish_non_exhaustive()
+    }
+}
+
 // WARNING:
 // As long as this struct is used through its `impl`, it is UnwindSafe.
 // (If `get_mut` is called on `inner`, unwind safety may get broken.)
@@ -161,6 +168,7 @@ impl rwh_06::HasDisplayHandle for ActiveEventLoop {
     }
 }
 
+#[derive(Debug)]
 pub struct EventLoop {
     /// Store a reference to the application for convenience.
     ///