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,

Bug: P670