Notification: Prevent showing a closed window

If a notification was closed, the connection will now be dead. To
prevent inconsistencies between when a user closes a notification and
when an application closes an applicated, check if the notification has
been closed before allowing any action.

Userland/Libraries/LibGUI/Notification.h

@@ -60,6 +60,7 @@ private:
 
     NonnullRefPtr<NotificationServerConnection> m_connection;
     bool m_showing { false };
+    bool m_disposed { false };
 };
 
 }