Support additional sources for label-display

[johanwk]

ELOT will pick up labels for resources defined in the file, as of 405971f.

Add an option to include more sources of identifier/label/attribute lists. Initially, ensure that a premade sparql query can be used to collect content from an endpoint or ontology file.

It's important to allow for several different sources that are merged.

[johanwk]

As of 1f5f697, ELOT will display labels for puris in any buffer. This requires retrieving labels first.

Example: retrieve labels from a local endpoint with bfo-core loaded, and from the QUDT public endpoint.

(elot-retrieve-labels-plist "http://localhost:3030/bfo-core/query" "~/tmp/bfotest.el")
(elot-retrieve-labels-plist "https://www.qudt.org/fuseki/qudt/sparql" "~/tmp/qudttest.el")

This produces files ~/tmp/bfotest.el and ~/tmp/qudttest.el containing lists of identifiers and labels, plus some annotations.

To load these for displaying labels, they need to be added to the variable `elot-slurp-global', like this:

(elot-read-slurp-global "~/tmp/bfotest.el" "~/tmp/qudttest.el")

Then to turn label display on, do M-x elot-label-display-setup.

There are some issues with regexps being too long, which can perhaps be fixed.

[johanwk]

There's also a useful lookup function, elot-label-lookup, which works even if fontification fails.