[ttml2] embedded content, background image styling, other edits

I've updated the TTML2 ED [1] a few times in the last few days to:

   - add embedded content element types (new section 9)
      - audio
      - chunk
      - data
      - font
      - image
      - resources
      - source
   - add background image styling
      - tts:backgroundImage
      - tts:backgroundRepeat
      - tts:backgroundPosition
   - sub-divide former parameters section into profiles section and
   parameters section

For detailed diffs, see change sets [2][3][4][5][6].

[1] https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html
[2] https://dvcs.w3.org/hg/ttml/rev/d16d284100b9
[3] https://dvcs.w3.org/hg/ttml/rev/67327764d375
[4] https://dvcs.w3.org/hg/ttml/rev/25035c814da5
[5] https://dvcs.w3.org/hg/ttml/rev/cc2b7aad7e7a
[6] https://dvcs.w3.org/hg/ttml/rev/e3fdbceb09cb