Only build reference search index when it's been changed

Using the same tricks as ve.ce.MWReferenceListNode... Like listening to list
node updates, which will probably catch unrelated changes? I'm not sure about
this.

Bug: 68890
Change-Id: Id9443c9574063933bc8fb35d09ab8b80b69bcd44
Merge "Only build reference search index when it's been changed"