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/web/event_loop/window_target.rs

@@ -67,6 +67,7 @@ impl<T> WindowTarget<T> {
                         virtual_keycode,
                         modifiers,
                     },
+                    is_synthetic: false,
                 },
             });
         });
@@ -83,6 +84,7 @@ impl<T> WindowTarget<T> {
                         virtual_keycode,
                         modifiers,
                     },
+                    is_synthetic: false,
                 },
             });
         });