Class WeakEvent

object --+
         |
        WeakEvent

Usage:

>>> myEvent = Event() # a, b
>>> callback1 = lambda a, b: dostuff(a, b)
>>> callback2 = lambda a, b: otherstuff(a, b)
>>> callback3 = self.callback3
>>> myEvent += callback1
>>> myEvent += callback2
>>> myEvent(a, b) # calls both callbacks
>>> myEvent -= callback1
>>> myEvent -= callback2
>>> myEvent(a, b) # no callbacks; does nothing

note: keep a ref somewhere, even to self.whatever, or it will be unlinked; see Reffer