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/windows/event_loop/runner.rs

@@ -4,7 +4,7 @@ use std::collections::VecDeque;
 use std::rc::Rc;
 use std::sync::{Arc, Mutex};
 use std::time::Instant;
-use std::{mem, panic};
+use std::{fmt, mem, panic};
 
 use windows_sys::Win32::Foundation::HWND;
 
@@ -40,6 +40,14 @@ pub(crate) struct EventLoopRunner {
     panic_error: Cell<Option<PanicError>>,
 }
 
+impl fmt::Debug for EventLoopRunner {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("EventLoopRunner")
+            .field("thread_msg_target", &self.thread_msg_target)
+            .finish_non_exhaustive()
+    }
+}
+
 pub type PanicError = Box<dyn Any + Send + 'static>;
 
 /// See `move_state_to` function for details on how the state loop works.