[#2959] Re-index datasets when group name changes.

The fix for this adds the Group domain objects to the stream of "listened to" objects that IDomainObjectModification implementers will be notified of.  I can't see this being a problem, but someone may know of a good reason.

The reason for indexing at this point, rather than within in the "group_update" action is that it's not possible to update the index whilst still adhering to the `context['defer_commit']` check in "group_update".