add `label-display.el` to show labels for uninformative identifiers

[johanwk]

The new example bfo-core.org contains many axioms that involve OBO identifiers, like this:

 - Domain :: obo:BFO_0000015 or obo:BFO_0000020

These are not readable, of course.

Add to ELOT the library label-display.el, which adds Emacs text properties to toggle between seeing labels and identifiers:

Also ensure marginalia is used for selection of identifiers. When looking for an identifier, the selection list should show

• [x] the label
• [x] the OWL type (Class, OP, DP, AP)
• [x] the source (from which ontology, or maybe endpoint)
• [x] the first words from the skos:definition or similar

Also ensure

• [x] lookup tables are buffer-local
• [x] there's an easy way to provide regexp for matching labels
• [ ] easy way to manage sources of id/label/metadata collections

[johanwk]

New file elot-label-display added 9627855. Not yet automatically loaded.

To use:

• with an ELOT file open, M-x load-file and select elot-label-display.el
• M-x elot-label-display-setup to ingest declarations
• press F5 to toggle between seeing labels and puris
• M-x elot-label-lookup to search for resources with completion

[johanwk]

With 405971f, label-display is automatically loaded, and the F4 menu has an entry with key r for looking up and inserting identifiers.

[johanwk]

@VladimirAlexiev, maybe you could test this new feature, to verify that it works OK?