Avoid creating unnecessary event loops

The `exec()` method will create another event loop and transfer
control over there which might introduce unexpected bugs.

src/gui/mainwindow.h

@@ -181,7 +181,7 @@ private slots:
 #if defined(Q_OS_WIN) || defined(Q_OS_MAC)
     void checkProgramUpdate();
 #endif
-    void toolbarMenuRequested(QPoint);
+    void toolbarMenuRequested(const QPoint &point);
     void toolbarIconsOnly();
     void toolbarTextOnly();
     void toolbarTextBeside();