Matroska tag handling improvement
Two improvements to matroska tag handling:
If the tracks element was parsed from the SeekEntry, don't
parse it a second time and recreate tracks, as this
loses any tags that were read using the seek table.
If a genuinely new Tracks element is found, do read that
as it is needed for MSE support.
Second, for the muxer store any Title tag as the track name, so that it shows up as the human readable name in players.