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/platform_impl/linux/x11/util/mod.rs

@@ -9,6 +9,7 @@ mod geometry;
 mod hint;
 mod icon;
 mod input;
+pub mod keys;
 mod memory;
 pub mod modifiers;
 mod randr;