'Register a new callback that will be called after this
``CanvasWidget`` is dragged.  This implicitly makes this
``CanvasWidget`` draggable.
:type callback: function
:param callback: The callback function that will be called
whenever this ``CanvasWidget`` is clicked.  This function
will be called with this ``CanvasWidget`` as its argument.'

def bind_drag(self, callback):

    self.__draggable = 1
    self.__callbacks['drag'] = callback
