Add current+previous value to property-related observation events

I know it's not part of the JCR spec, but it would be super helpful if the JcrEvent had access to the before and after state of a property (for property related events). Sort of surprising that it's not part of the spec, actually.

For reference: https://github.com/ModeShape/modeshape/blob/master/modeshape-jcr/src/main/java/org/modeshape/jcr/JcrObservationManager.java#L827

This would be particularly useful when using Observation to do transparent auditing of changes (my particular use case).