Policy that is raised for each content property change. Any policy implementations must be aware
that the transaction in which this is called could still roll back; no filesystem changes should
occur against the source content until after the transaction has successfully completed.
propertyQName - the name of the property that changed
beforeValue - the value of the content data prior to the change.
Note that this value may be null or any of it's member
values may be null according to the contract of the
ContentData class.
afterValue - the value of the content data after the change