Exposing DASH text tracks as cues

Text for exposing DASH text tracks as Cues - based on info from DASH IF, DASH spec and ISO 14496-30. However, it seems this area is evolving rapidly so it could use feedback from whatever DASH experts we can find.

BTW, this PR refers to text in the previous PR "ISOBMFF text tracks and captions"
For non-existent cue types, we should always add:
"Alternatively, browsers can also map the xxx features to WebVTTCue objects. Finally, browsers that cannot render xxx captions should expose them as DataCue objects [[HTML5]]. In this case..."
As we do for CEA708.