data model: factor out register_item_def, ret new item def
data model: factor out register_item_def, ret new item def

Limecraft/ebu-libmxf,Limecraft/ebu-libmxf,stuarthicks/libmxf,stuarthicks/libmxf,stuarthicks/libmxf,Limecraft/ebu-libmxf,stuarthicks/libmxf