Fixes to displaying literals in RDF exports.

* [GV_FILE,FIX] Remove the temporary file when the file conversion is finished.
* [RDF_GRAPH_THEORY,FIX] rdf_vertex_check/2 would incorrectly include literals that are not in the preferred language tag.
* [RDF_GRAPH_THEORY,FIX] rdf_vertex_check/2 was not prepared for non-label literals.
* [RDF_READ,NEW] Added rdf_preferred_literal/5.
* [RDFS_READ,EXT] Rewrote rdfs_preferred_label/3 to .../4, using the new rdf_preferred_literal/5.
Replaced the DBNL code to a separate project.