Generic Listener

[Mike Kestner <mkestner ameritech net>]

I checked in a generic listener interface last night.  It has one
method, Listener::event, which emits an event_notify signal on the
listener object.  The signal delivers a BonoboArg parameter passed into
the method.

I'll be using it initially to add change_listener functionality to
Properties and Bags.

Regards,
Mike