'If this ``CanvasWidget`` has a drag callback, then call it;
otherwise, find the closest ancestor with a click callback, and
call it.  If no ancestors have a click callback, do nothing.'

def __click(self, button):

    if (button in self.__callbacks):
        cb = self.__callbacks[button]
        cb(self)
    elif (self.__parent is not None):
        self.__parent.__click(button)
