Allow rollback on CancelableEvents in case of cancelling

Currently if several listeners are called on a CancelableEvent and this one is cancelled at a point, the previous listeners are never rolled-back.
This could cause some inconsistencies in the wiki.

We propose to add a new method in CancelableEvent interface:

onCanceled(CancelableEvent event, Object source, Object target)

that the events could implement to apply the rollback. The ObservationManager should then call those rollbacks methods in case of a cancelled, on the previously applied listener.