[Qt] disable tray interactions when client model set to 0

- this prevents the ability to fiddle around with the system tray when
  already shutting down (e.g. on slow shutdowns because of a proxy delay)
- extends solution for #4360

src/qt/bitcoingui.h

@@ -103,6 +103,7 @@ private:
     QAction *showHelpMessageAction;
 
     QSystemTrayIcon *trayIcon;
+    QMenu *trayIconMenu;
     Notificator *notificator;
     RPCConsole *rpcConsole;