Add saving subtitle track name for MatroskaExtractor

[vadimdolgachev]

Some files have several subtitles in the same language, such as forced and regular subtitles. In this case, a name is added for the subtitle track.
This code allows to save the name of the subtitle track. DefaultTrackNameProvider consider the name for TRACK_TYPE_TEXT track type.
The subtitle name and language name in the pair can also be seen in VLC player.
Sample video with subtitle track name: https://drive.google.com/file/d/1WoWcL9B4cVd1fsDEAtYpXDmG0cMF0F_3/view?usp=sharing

[icbaker]

Thanks for the contribution! Using the name/title from Matroska files to populate Format.label looks good.

But the changes to DefaultTrackNameProvider cause us to surface an un-localisable string (directly from the media file) in an otherwise localised context. It's also not clear the concatenating language & track name always makes sense - especially as in some cases the track name might contain the language so it would be duplicated.

So I'm going to merge only the changes to MatroskaExtractor - it should be possible for you to keep the track name behaviour in your own app by providing a custom TrackNameProvider. Hope that's OK!

[vadimdolgachev]

Good. Thanks!