'Bind fun to mouse-click event on canvas.
Arguments:
fun -- a function with two arguments, the coordinates of the
clicked point on the canvas.
num -- the number of the mouse-button, defaults to 1
Example (for a TurtleScreen instance named screen)
>>> screen.onclick(goto)
>>> # Subsequently clicking into the TurtleScreen will
>>> # make the turtle move to the clicked point.
>>> screen.onclick(None)'

def onclick(self, fun, btn=1, add=None):

    self._onscreenclick(fun, btn, add)
