add an attribute to <keyword>, to indicate the source vocabulary

[bansp]

This is a future-oriented issue, for when we are able to draw at least a subset of keywords from some controlled vocabularies.
(Thanks to @raspberryjoy for the inspiration.)

[bansp]

Exploring a further hint from Sasa, I got to several perma links and stopped, because there's more, but I don't have a clear idea of how to use them in the SIS:

• https://op.europa.eu/s/z1au
• https://op.europa.eu/s/z1av
• https://op.europa.eu/s/z1aw
• https://op.europa.eu/s/z1ax (concept scheme for "file type", this one is comprehensive, but, again: how can we utilize it?)

This is the overall query I used: https://op.europa.eu/en/web/eu-vocabularies/search-results?p_p_id=eu_europa_publications_portlet_search_result_summary_SearchResultSummaryPortlet_INSTANCE_XrSAFTsHKi9q&p_p_lifecycle=1&p_p_state=normal&p_p_mode=view&queryText=file%20format&facet.collection=EUVoc&sortBy=RELEVANCE-DESC&SEARCH_TYPE=SIMPLE&QUERY_ID=363446074&&resultsPerPage=50

[bansp]

An unexpected result of this ticket was my commit 7612f3b , containing a potential base for format cross-categorisation.

[bansp]

I'm going to add a label SIS:keywords, because that area may grow into a separate subproject with numerous subtasks.

And I'll keep this issue as schema-oriented, possibly transferring the links above to a more appropriate issue. I feel that, myself, I can't deal with the op.europa.eu proposals and remain reasonably sane. Need to spread the load.

[bansp]

I've added content completion for a subset of GDFR keywords. At this point, it is possible to have bare <keyword> elements with unconstrained content, while <keyword type="gfdr"> makes it possible to choose from a closed list.
Unfortunately, at least for now, I was not able to make oXygen suggest the @type attribute after "<keyword " has been entered -- "type='" has to be typed letter by letter, ignoring validation issues (they stop when a value has been selected from the drop-down).
I'm afraid I'm very far from expert in XML Schema, and assistance with the implementation of this new, fancier <keyword> element would be appreciated.