JSON textunits should have a global ID

The textunit resIds for JSON are generated with contentHash, which means the TextFlowTargets for:

will be the same.
This may not be desirable, as there is no context or convention that says they are they same in the way of translatable text.

There should be a means of getting the global id, e.g. thing.title, and using that in the resID.
See test JSONAdapterTest:testDuplicateContent.