Group audio / subtitle tracks together via matroska-v5

linking audio tracks to specific subtitle tracks is an upcoming MKV container feature so i thought i’d put the suggestion here for now so people can like this post if they want to be able to group audio + subtitle tracks (like when there are multiple commentaries)

this PR explains the spec: add a list of tracks to group them together by robUx4 · Pull Request #746 · ietf-wg-cellar/matroska-specification · GitHub