Support rdf:langString in API
While rdf:langString is supported somewhat, this support is reserved to SPARQL inserts containing language tagged strings as literals, and the related builtin SPARQL functions (STRLANG/LANGMATCHES). There's notably no support for retrieving langstrings, using them in statements, or producing/reading RDF data containing them.
This MR adds 2 new public API calls:
- tracker_sparql_statement_bind_langstring()
- tracker_sparql_cursor_get_langstring()
And makes rdf:langString fully supported across the board, on all connection/cursor types and produced/read across all RDF formats.