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.

Userland/guitest2.cpp

@@ -40,6 +40,7 @@ int main(int argc, char** argv)
 #endif
 
     auto* launcher_window = make_launcher_window();
+    launcher_window->set_should_exit_app_on_close(true);
     launcher_window->show();
 
     return loop.exec();