Hbbtv mpeg2ts 5

This is pull request 7 out of 9 and it contains updates proposed by HbbTV for the section on MPEG2-TS. See the [visual diff to silviapfeiffer/HTMLSourcingInbandTracks:master](http://services.w3.org/htmldiff?doc1=https%3A%2F%2Frawgit.com%2Fsilviapfeiffer%2FHTMLSourcingInbandTracks%2Fmaster%2Findex.html&doc2=https%3A%2F%2Frawgit.com%2Fc-alpha%2FHTMLSourcingInbandTracks%2Fhbbtv-mpeg2ts-5%2Findex.html) for a better overview than the diffs. Please ignore the (many) individual diffs contained in this request, and focus on the diffs (source and visual) alone.

**Change notes and rationale:**
* Added new columns to make room for DVB and ISDB in the table for mapping text track content into metadata text track cues.
* Added rules on populating the cue attributes for DVB TS.
* ISDB needs to be filled in by someone knowledgeable.
surely, cues without content are pretty useless?