Add the actor's methods to the callback registry.

def plug(self):
        '''Add the actor's methods to the callback registry.'''
        if self.__plugged:
            return
        for _, method in inspect.getmembers(self, predicate=inspect.ismethod):
            if hasattr(method, '_callback_messages'):
                for message in method._callback_messages:
                    global_callbacks[message].add(method)
        self.__plugged = True