X11: Sync key press/release with window focus (#1296)

* 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

* Clarify code with comments

src/event.rs

@@ -132,6 +132,15 @@ pub enum WindowEvent {
     KeyboardInput {
         device_id: DeviceId,
         input: KeyboardInput,
+        /// 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
+        ///   when a window gains focus. Likewise, synthetic key release events
+        ///   are generated for all keys pressed when a window goes out of focus.
+        ///
+        /// Otherwise, this value is always `false`.
+        is_synthetic: bool,
     },
 
     /// The cursor has moved on the window.