Add common parent type of OBJECT_VERSION_ID and HIER_OBJECT_ID

Currently, there is at least one place in the model where either OBJECT_VERSION_ID or
HIER_OBJECT_ID should be allowed, namely LOCATABLE.uid. There are probably others.
Secondly, both of these identifier types are UID-based, with an extension of some
kind, and from the modelling point of view should be based on a common intermediate
supertype.