Small improvements to displaying RDF graphs.

* [GV_DCG,SC] Simplified graph attributes code by using module OPTION_EXT.
* [GV_DCG,EXT] Added graph attribute overlap=false.
* [RDF_EXPORT,EXT] Changed the way in which RDF edges are colored.
* [RDF_GRAPH_THEORY,EXT] Exclude simple literals that have a plain literal equivalent.
* [RDF_TERM,NEW] Added rdf_is_[plain,simple]_literal/1.
* [RDFS_VOC,NEW] Re-added rdf_voc_[pdf,web]/1.