'Start a drag from button'

def __startDrag(self, button):

    action = button.defaultAction()
    desc = action.data()
    icon = action.icon()
    drag_data = QMimeData()
    drag_data.setData('application/vnv.orange-canvas.registry.qualified-name', desc.qualified_name.encode('utf-8'))
    drag = QDrag(button)
    drag.setPixmap(icon.pixmap(self.iconSize()))
    drag.setMimeData(drag_data)
    drag.exec_(Qt.CopyAction)
