User:GWicke/DOM structure development

Possible updates for Parsoid/RDFa_vocabulary.

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 * rename data-gen to data-special (again!) since it is more about special semantics than generated content. Alternative: data-mwsem or data-mw?
 * rename data-mw to data-rt (round trip). Private to Parsoid; not necessarily in JSON format.
 * add data-attribs, a JSON encoding of non-semantic (presentational) attributes. Public and editable. Alternative names: data-json or data-attribs-json?
 * represent semantically interesting information as RDFa attributes:
 * resource : The canonical URL of the used image (image page)
 * caption: The caption belonging to this image. Also implied by figcaption really, so questionable if needed.
 * Look at http://schema.org/ImageObject, and perhaps map our semantic attributes to those