Add slot for skos:prefLabel to ThingMixin

[mih]

This will serve the same purpose as the shacl annotation for a class/slot name. However, this approach also extends into the realm of individual instances.

Use case: Role instances need to be presented in a meaningfully compact way (also Instruments, etc). skos:prefLabel for https://id.loc.gov/vocabulary/relators/spn.html could simply be set to Sponsor and would offer the information to do this.

[mih]

#268

[mih]

I think this is not thinking far enough. #266 is another case like this.

I think it is actually calling for something like meta_properties, with a dedicated (inlined) record that collects all this information on the metadata record itself.

[mih]

Done differently via #269