'Bind fun to key-release event of key.
Canvas must have focus. See method listen'

def _onkey(self, fun, key):

    if (fun is None):
        self.cv.unbind(('<KeyRelease-%s>' % key), None)
    else:
        def eventfun(event):
            fun()
        self.cv.bind(('<KeyRelease-%s>' % key), eventfun)
