Add default right-click menu

1. Set default menu

 this->setContextMenuPolicy(Qt::ContextMenuPolicy::DefaultContextMenu);

2. Rewrite the virtual function

void contextMenuEvent(QContextMenuEvent *event);

3. Example

// Constructors 
//this->setContextMenuPolicy(Qt::ContextMenuPolicy::DefaultContextMenu);
   widgetMenu=new QMenu(this);
   QAction *actNextofWidgetMenu=new QAction(" next ",widgetMenu);
   QAction *actForwardofWidgetMenu=new QAction(" Previous ",widgetMenu);

   widgetMenu->addAction(actNextofWidgetMenu);
   widgetMenu->addAction(actForwardofWidgetMenu);

   connect(actNextofWidgetMenu,&QAction::triggered,this,&Widget::on_actNextofWidgetMenu);
   connect(actForwardofWidgetMenu,&QAction::triggered,this,&Widget::on_actForwardofWidgetMenu);