Add support for qualifiers
The Wikibase data model not only supports properties,
but also qualifiers ('properties on properties').
It makes sense for inteGraality to also support qualifiers.
- Extend PropertyConfig object to support additional fields qualifier and value
- Add methods get_qualifier_info, get_qualifier_info_no_grouping and get_totals_for_qualifier which run slightly different SPARQL queries.
- Add syntax support in the template configuration parser: we just reuse the Listeria syntax:
- Pxxx/Pzzz, for property Pxxx with a Pzzz qualifier
- Pxxx/Qyyy/Pzzz, for Pxxx linking to an item Qyyy with a Pzzz qualifier,