'Bind fun to mouse-click event on this turtle on canvas.
Arguments:
fun --  a function with two arguments, to which will be assigned
the coordinates of the clicked point on the canvas.
num --  number of the mouse-button defaults to 1 (left mouse button).
add --  True or False. If True, new binding will be added, otherwise
it will replace a former binding.
Example for the anonymous turtle, i. e. the procedural way:
>>> def turn(x, y):
...     left(360)
>>> onclick(turn)  # Now clicking into the turtle will turn it.
>>> onclick(None)  # event-binding will be removed'

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

    self.screen._onclick(self.turtle._item, fun, btn, add)
    self._update()
