Add a simple close button ("X") to windows.

Clicking the button generates a WindowCloseRequest event which the client app
then has to deal with. The default behavior for GWindow is to close() itself.

I also added a flag, GWindow::should_exit_event_loop_on_close() which does
what it sounds like it does.

This patch exposed some bugs in GWindow and GWidget teardown.

Kernel/GUITypes.h

@@ -66,6 +66,7 @@ struct GUI_Event {
         KeyUp,
         WindowActivated,
         WindowDeactivated,
+        WindowCloseRequest,
     };
     Type type { Invalid };
     int window_id { -1 };