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.
"If the duration of an event is defined, it must be at least 250ms."