'Bind fun to mouse-button-release 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).
Example (for a MyTurtle instance named joe):
>>> class MyTurtle(Turtle):
def glow(self,x,y):
self.fillcolor("red")
def unglow(self,x,y):
self.fillcolor("")
>>> joe = MyTurtle()
>>> joe.onclick(joe.glow)
>>> joe.onrelease(joe.unglow)
### clicking on joe turns fillcolor red,
### unclicking turns it to transparent.'

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

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