Added debug tool for threads.

* [DCG_CONTENT,FIX] Added determinism to arrow//2.
* [HTML_TABLE,EXT] Added the header_column/1 option; renaming the header/1 option to header_row/1 everywhere.
* [RDF_WEB,SC] Removed rdf_graphs_web/1 and rdf_namespaces_web/[1,2]. We're superseeding these DOM-based predicates with DCG rules.
* [WEB_THREADS,NEW] New module providing a Web page overview of the currently running threads.