Ability to register callback to recieve worker change events

[cohix]

Currently, a Runnable includes the OnChange method which allows it to handle events such as adding new instances.

With the Superfunc concept, this method is no longer available, but in order for this to be viable for stateful Runnables, they need to be able to repond to these same events.

I propose that callbacks can be registered, either using traditional functions or via a Grav pod, to allow a callback to respond to these change events.