Notification: Use RefPtr for connection

With this RefPtr, we can initialize the connection to the
NotificationServer upon showing the notification. With this, we can
prevent double shows and updates or closes before showing.

Userland/Libraries/LibGUI/Notification.h

@@ -74,9 +74,7 @@ private:
     RefPtr<Gfx::Bitmap> m_icon;
     bool m_icon_dirty;
 
-    NonnullRefPtr<NotificationServerConnection> m_connection;
-    bool m_showing { false };
-    bool m_disposed { false };
+    RefPtr<NotificationServerConnection> m_connection;
 };
 
 }