DASH: implement forced-subtitles

[peaBerberian]

This PR implements DASH forced-subtitles that are subtitles defined for when no other subtitle track is selected.

The idea is often to implement "Forced Narrative Subtitles" which allows to provide clarifications on different element in the screen. You can read more information on it here for example: https://partnerhelp.netflixstudios.com/hc/en-us/articles/217558918-Understanding-Forced-Narrative-Subtitles

With this PR, those text tracks will now have a forced attribute set to true in the various APIs where it is inspectable (getAvailableTextTracks, getTextTrack and the events textTrackChange and availableTextTracksChange).

Moreover, they are used as default text tracks, instead of no text track, when no set preference match the current choice, following a specific rule:

• If there's a language associated to this forced subtitles, it is only set if this is the same language than for the used audio track.
• If no forced subtitle is found associated to the language of the current audio track but a language-less forced subtitles track exists, then that one
• In any other case, no subtitles are chosen by default.

This algorithm was chosen after pondering what the most logical would be. It is imperfect however (setting forced subtitles by default even if it is in a different language than the audio track could be preferred for example) and as such it may be changed in the future.
Though in any case, that track can be manually set by setTextTrack or preferences, which thus can be used for more precize settings.