Support new dynamic list of DASH IF test vectors in reference player

Created by: wilaw

The DASH IF maintains a repository of test vectors at http://testassets.dashif.org/. Currently, the dash.js reference player contains a subset of these vectors that was manually copied over about two years ago and is a static part of sources.json. Many are stale and the UI in the reference player is poor.

The test assets DB now exports the test vectors in a JSON format https://testassets.dashif.org/dashjs.json specifically designed to match the pull-down menu system implemented in the reference player.

This issue requests:

1. Modify the reference player STREAM selector to dynamically pull-in the data from http(s)://testassets.dashif.org/dashjs.json at load time.
2. The menu should show the hierarchy of test sections. 1st level should show "DASH Industry Forum test vectors", 2nd should show the sub-headings in the JSON (for example "Single Resolution Multi-Rate", "Multi-Resolution Multi-Rate" etc), 3rd level would list the vectors for that section.
3. Ensure the loading user-experience is not negatively affected. You may want to only load in the remote sources if the user selects "DASH Industry Forum test vectors" from the stream menu.