ENH: Enables use of NotObjectId in Interactive PDF Segmenter

Enabled by restricting objectId to only one value. Any label which
is not objectId or voidId is now NotObjectId.

Change-Id: I008c55632188c36b4f3c9b3a388d04cb54cd5db4
Merge topic 'interactively-enforce-only-one-objectId'

54b4170 ENH: Enables use of NotObjectId in Interactive PDF Segmenter