css.rdf.XDocumentRepository: method getStatementRDFa() also returns a bool.
 librdf_repository:
   setStatementRDFa() no longer generates the rdfs:label statement.
   getStatementRDFa() no longer handles the rdfs:label statement.
   add a set to store elements with xhtml:content.
 RDFaExportHelper:
   adapt export to new getStatementRDFa() and lack of rdfs:label statement.