simpler synchronization logic with GModel: we now keep track of all internal...

simpler synchronization logic with GModel: we now keep track of all internal entities that have been unbound
and we always remove them from GModel

We already did the same thing if a new entity was later re-bound to the tag; but we failed to do it if the *same* entity is rebound.
Since we don't guarantee that the subshapes will stay the same, we need to regenerate the GEntity.