Delete DASH tracks on reset

Created by: frapontillo

If I try to change the DASH stream, all the tracks added by dash.js should be removed. This is the same problem as reported in #151 (closed). #245 confirmed a fix that just included the removal of clues, which has the same visual effect, but it breaks many custom players that allow users to select between a list of captions/subtitles.

Also in #245, the tracks are accessed as an array, but Chrome 42 uses an item(index) getter.

I have lost a couple of hours looking to hack around and manually delete track items, but it looks like it's not possible right now. Are you in a position to push for a spec change?