Memory leak caused by the default content menu of QMainWindow

Right click on the menubar or docked widget several times, the default context menu will be shown one by one. When the new menu is shown, the old one does not destroyed. This will lead to memory leak.

You can reproduce the problem with following code:

• right click on the menubar or docked widget serveral times
• then click the "dump object tree" button, you will see many menu objects in console window.

  #include <QtGui>
  class MainWindow : public QMainWindow
  {
      Q_OBJECT
  public:
      explicit MainWindow(QWidget *parent = 0);
  private slots:
      void onButtonClicked();
  };
  
  MainWindow::MainWindow(QWidget *parent)
  {
      addToolBar("ToolBar");
      addDockWidget(Qt::LeftDockWidgetArea, new QDockWidget("DockWidget"));
      QPushButton * btn = new QPushButton("dump object tree");
      setCentralWidget(btn);
      connect(btn, SIGNAL(clicked()), SLOT(onButtonClicked()));
  }
  
  void MainWindow::onButtonClicked()
  {
      dumpObjectTree();
  }
  
  #include "main.moc"
  int main(int argc, char *argv[])
  {
      QApplication a(argc, argv);
      MainWindow w;
      w.show();
      return a.exec();
  }