Refactor text tracks

[bbert]

This PR refactors text tracks managements in order to:

• keep only type "text" for all text tracks (removed 'fragmentedText' and 'embeddedText' types)
• rely on TextTrackInfo.isFragmented and isEmbedded to get type of text tracks
• unify text tracks metadata returned by MediaPlayer.getTracksFor() and in TEXT_TRACKS_ADDED event (TextTrackInfo extends MediaInfo)
• rely on segments templates to know if a text track is fragmented or not

As a benefit:

• you can get all text tracks using MediaPlayer.getTracksFor('text') agnostically to the type of text tracks (fragmented or not, or embedded) and then ease the managements text tracks of different types in the same stream
• the list of embedded text tracks are therefore availaible using MediaPlayer.getTracksFor('text'), this was not the case before
• media settings preferences (language, role) are then persistent accross all types of text tracks
• fragmented vtt text tracks without mp4 format are supported