Stabilizaton of basic SVG_DCG and XML_DCG codebase, featuring:
  * Lists of DCG attributes.
  * Lists of DCG entities.
  * Collection of multiple subtrees through variable injection.
  * Setting of XML namespace though DCG rule injection.

* [DCG_CONTENT,SC] Relocated dcg_void//0 (renamed to void//0) from module DCG_GENERIC.
* [DCG_GENERIC,SC] Removed the overly specific predicates dcg_list//[3,4].
* [SVG_DCG,EXT] Finished rewriting of existing predicates, making use of the adapted XML_DCG.
* [SCG_DCG,DEB] Added unit test for rectangle entity.
* [XML_DCG,NEW] Added xml_entities//3 for processing multiple DCG rules, keeping the trees in a list and setting a uniform namespace.
* [SVG,SC] Unified with SVG_DCG.
* [SVG_ATTRIBUTES,EXT] Added rules for base_profile, content_script_type, content_style_type, preserve_aspect_ratio, and zoom_and_path.
* [SVG_DATAYPES,EXT] Added support for simple MIME types (more of a stub).
* [SVG_ENTITIES,NEW] New module for SVG entity DCG rules.