[agiowt] (1) Split more WebVTT-specific things into the WebVTT spec. This also makes some normative changes to HTML for handling non-WebVTT cue types, but that shouldn't affect any existing implementations.
Affected topics: HTML, Microdata, Rendering, Security, Video Text Tracks

git-svn-id: http://svn.whatwg.org/webapps@7748 340c8d12-0b0e-0410-8428-c7bf67bfef74