[RDFa] ISSUE-34: partial resolution to @href everywhere

Hi all,

Today, we agreed that we would support @href *and* @resource everywhere,
with the following note:

- @href may become navigable/clickable in a future version of XHTML
- we encourage X/HTML WGs to *not* make @resource clickable/navigable.
- from an RDFa standpoint, they both determine the object of a triple
(or the subject under a @rev).

The question, then, is what to do when *both* @resource and @href
appear.

I have a proposal, which I think makes sense and provides solid
justification for having both attributes: @resource overrides @href just
like @content overrides the content of an element.

Effectively, this joins up with Mark's reasoning that @resource is used
when the navigable link is not really the semantic link. If there is a
clickable and semantic link, use @href. If there is only a semantic
link, use @resource. If there's both but they're not the same, use both
attributes accordingly.

Thoughts?

-Ben