Fix unresponsive GUI on quit if a polling operation is in progress

app/backend/computermanager.h

@@ -161,6 +161,8 @@ signals:
     void quitAppCompleted(QVariant error);
 
 private slots:
+    void handleAboutToQuit();
+
     void handleComputerStateChanged(NvComputer* computer);
 
     void handleMdnsServiceResolved(MdnsPendingComputer* computer, const QHostAddress& address);