Use various RDFa types for links
- the used RDFa types for links are now identical to those listed in http://www.mediawiki.org/wiki/Parsoid/RDFa_vocabulary, and are supported for serialization
- Editors are responsible for adjusting the type when converting between link types. Adding a caption to an mw:UrlLink for example should convert it into an mw:ExtLink.
Update: rebased on top of trace patches