Prevent recursions when triggering menus in QToolButton

With a global shortcut set it would be possible
to let the button re-open the menu again and again,
each time spinning an event loop.

Task-number: QTBUG-30399
Change-Id: If7eddc115c77fef3df3e751fd72e7414cedaf272
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>

src/widgets/widgets/qtoolbutton.cpp

@@ -680,6 +680,10 @@ void QToolButton::showMenu()
         d->menuButtonDown = false;
         return; // no menu to show
     }
+    // prevent recursions spinning another event loop
+    if (d->menuButtonDown)
+        return;
+
 
     d->menuButtonDown = true;
     repaint();