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/linux/x11/ime/mod.rs

@@ -5,6 +5,7 @@ mod context;
 mod inner;
 mod input_method;
 
+use std::fmt;
 use std::sync::mpsc::{Receiver, Sender};
 use std::sync::Arc;
 
@@ -56,6 +57,12 @@ pub(crate) struct Ime {
     inner: Box<ImeInner>,
 }
 
+impl fmt::Debug for Ime {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Ime").finish_non_exhaustive()
+    }
+}
+
 impl Ime {
     pub fn new(
         xconn: Arc<XConnection>,