[RDFa] ISSUE-3 @class and @role for rdf:type

Another issue up for discussion.

ISSUE-3
http://www.w3.org/2006/07/SWD/track/issues/3

The question is what @class and @role should yield in XHTML1.1+RDFa. We
don't have complete consensus on this (we specifically note Steven
Pemberton's worries about the reuse of the @class attribute), but the
current solution, as accepted in the Primer and in many use cases, is as
follows:

@class yields rdf:type only if the value is namespace-qualified. @class
contains a space-separated list of values. Only those values which are
namespace-qualified yield rdf:type triples.

@role does not exist in XHTML1.1, so it is not used here. In XHTML2, it
is expected to yield a triple with predicate xhtml2:role.

+1 if you agree, otherwise email your disagreements and explanation.

-Ben