Implement the following events:
TextTrack: onload, onerror, oncuechange (the spec doesn't say, but I think these should propagate up to the <track> element as well, when applicable)
TextTrackCue: onenter, onexit
Also need to implement the 'addtrack' event.
All <track> related events have been implemented. All that is left to be done is to sort and filter them before dispatch, and that is covered by this bug: