Implement windows focus key press/release on Windows (#1307)

* X11: Sync key press/release with window focus

* When a window loses focus, key release events are issued for all pressed keys
* When a window gains focus, key press events are issued for all pressed keys
* Adds `is_synthetic` field to `WindowEvent` variant `KeyboardInput`
  to indicate that these events are synthetic.
* Adds `is_synthetic: false` to `WindowEvent::KeyboardInput` events issued
  on all other platforms

* Implement windows focus key press/release on Windows

* Docs

Co-authored-by: Murarth <murarth@gmail.com>

src/event.rs

@@ -150,9 +150,10 @@ pub enum WindowEvent {
         /// If `true`, the event was generated synthetically by winit
         /// in one of the following circumstances:
         ///
-        /// * **X11**: Synthetic key press events are generated for all keys pressed
+        /// * Synthetic key press events are generated for all keys pressed
         ///   when a window gains focus. Likewise, synthetic key release events
         ///   are generated for all keys pressed when a window goes out of focus.
+        ///   ***Currently, this is only functional on X11 and Windows***
         ///
         /// Otherwise, this value is always `false`.
         is_synthetic: bool,