Dispatch handling of this event to a receiver.

        This method will invoke ``receiver._session_callback_added`` if
        it exists.

def dispatch(self, receiver):
        ''' Dispatch handling of this event to a receiver.

        This method will invoke ``receiver._session_callback_added`` if
        it exists.

        '''
        super(SessionCallbackAdded, self).dispatch(receiver)
        if hasattr(receiver, '_session_callback_added'):
            receiver._session_callback_added(self)