'Register a new callback that will be called whenever this
``CanvasWidget`` is clicked on.
: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.
:type button: int
:param button: Which button the user should use to click on
this ``CanvasWidget``.  Typically, this should be 1 (left
button), 3 (right button), or 2 (middle button).'

def bind_click(self, callback, button=1):

    self.__callbacks[button] = callback
