Hbbtv dash

This is pull request 1 out of 9 and it contains the updates proposed by HbbTV for the section on MPEG DASH. 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-dash%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:**
* We are proposing to source DASH EventStreams as text tracks. These are e.g. used for ad insertion, and for dealing with timelines in second screen applications.
* The updates to the table for sourcing the attributes of text tracks other than event streams try to provide further detail and align with MPEG-DASH and DVB's profile of it.
* The updates to the table for sourcing the attributes of audio and video tracks try to provide further detail and align with MPEG-DASH and DVB's profile of it.
* At the end of the section, we propose a new table for sourcing the attributes of text tracks representing DASH EventStreams.
id is the field from the event message box ('emsg'), as are the other fields referenced in this column. Again, perhaps the column heading needs changing.