Create event listener framework within Cascade for all pluggable events

Cascade would see benefit in having a generic EventListener interface
that could be implemented and registered. When events are generated,
the listener itself would determine what it would want to do based on
the type of event. This would go a long way towards having a single way
to provide trigger-like capabilities without having specific, separate,
and numerous configuration frameworks for each of the different type of
listener type plugin interfaces already available (e.g. workflow
triggers, publish triggers, etc).