Allow multiple types to share idno SERIAL sequence

The multipart id numbering system supports two modes of sequence number generation via SERIAL elements: (1) a single sequence for the entire table (eg. all objects regardless of type) or (2) separate per-type sequences

A third option should be added, allowing specification of sequences that are shared across several selected types. This would allow a single sequence to be used for groups of related types (Eg. different types of photographic materials in a photo collection).