Added setters for ResourceEvent's message properties

The ResourceManager's behavior is to dispatch several ResourceEvents upon modifications of a resource (domain object).
Adding those setters permits to domain events listeners to dynamically change the messages properties that will eventually be used to generate a FlashBag or a log of some sort without the need to rewrite the DomainManager.
Merge pull request #1859 from hmlb/patch-1

Added setters for ResourceEvent's message properties