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/macos/event.rs

@@ -264,6 +264,7 @@ pub unsafe fn modifier_event(
                 virtual_keycode,
                 modifiers: event_mods(ns_event),
             },
+            is_synthetic: false,
         })
     } else {
         None

src/platform_impl/macos/view.rs

@@ -582,6 +582,7 @@ extern "C" fn key_down(this: &Object, _sel: Sel, event: id) {
                     virtual_keycode,
                     modifiers: event_mods(event),
                 },
+                is_synthetic: false,
             },
         };
 
@@ -633,6 +634,7 @@ extern "C" fn key_up(this: &Object, _sel: Sel, event: id) {
                     virtual_keycode,
                     modifiers: event_mods(event),
                 },
+                is_synthetic: false,
             },
         };
 
@@ -741,6 +743,7 @@ extern "C" fn cancel_operation(this: &Object, _sel: Sel, _sender: id) {
                     virtual_keycode,
                     modifiers: event_mods(event),
                 },
+                is_synthetic: false,
             },
         };