'Deregister a callback function.  If ``func`` is none, then
all callbacks are removed for the given event.'

def remove_callback(self, event, func=None):

    if (event is None):
        events = list(self._callbacks.keys())
    elif (event == 'select'):
        events = ['click1', 'space', 'return']
    elif (event == 'move'):
        events = ['up', 'down', 'next', 'prior']
    else:
        events = [event]
    for e in events:
        if (func is None):
            del self._callbacks[e]
        else:
            try:
                del self._callbacks[e][func]
            except:
                pass
