[ttml2] various

   - allow nested ttm:item
   - add missing cea708Transform* named items
   - add sourceFormat named item
   - refine {source,target}Format named items to take xsd:token or
   xsd:anyURI
   - add terminology on {audio,font,image} resource and content vs
   non-content image
   - improve image examples

[1] https://dvcs.w3.org/hg/ttml/rev/277f01f27bf7