Listener interface to receive notifications of changes in the current editing state.
Editing changes should only be thrown at major points (not at every model change event).
So this class can be used to drive database (or other slow) model updates. This can e.g. be done
by ignoring regular model changes and only updating the database when receiving editing state events.
This listener can be added to an ILspInteractivePaintableLayer in order to listen for editing changes.