Improved closed_captions compatibility with hlsjs-playback.

[bp2008]

• Fixed sync issues between closed_captions GUI and actual backend state when using hlsjs-playback.
• closed_captions menu now renders track names with suffix ' (forced)' if indicated by an optional boolean field tracks[i].attributes.forced (currently only provided by hlsjs-playback via a pull request I made earlier ).

This PR is meant to go along with my other PR in the hlsjs-playback repo but can also provide some small benefit when used standalone.