HLS.js and side-loaded subtitles: Minor issues

[torerikal]

The HlsjsVideoStreamer's text track implementation has some issues:

• There isn't a dedicated API for dealing with side-loaded subtitles in HLS.js. For now, the video element text track API is mainly used, also for displaying and controlling in-band subtitles added from the stream.
• Combining in-band (stream) subtitles with side-loaded subtitles is not supported.
• HLS.js subtitle track housekeeping doesn't work with side-loaded subtitles of type 'captions'. For now tracks with kind 'captions' are added as 'subtitles'.

If/when HLS.js gets support for side-loaded subtitles, these issues should be revisited.