Add subtitle->audio track linking

[rcombs]

Useful for cases where the appropriate tracks are otherwise ambiguous; see added notes.md section for a few examples.

Also clarified that TrackOverlay refers to tracks by their UIDs rather than their numbers; as far as I can tell this is the intent, but I can't find anything existing confirming this either way (nor any implementation code), so I could be wrong, but it seems consistent with other elements.

Previously it was suggested that TrackLink should go in a dedicated container element with maxOccurs=1, but I don't see any particular technical reason for that, and TrackOverlay already allows multiple instances without a container, though I can add one if that's a policy for newly-added elements or something.

[robUx4]

The TrackOverlay refers to the SilentTrack feature which itself has SilentTrackNumber. This is a track number.

Given there's probably 0 use anywhere in the wild of this feature I don't mind the change. We may also change the name to TrackUIDOverlay or OverlayTrackUID.

Now given all of these are unused we might as well mark them as deprecated so they are not used while not specified properly or are of any use to anyone. This feature is basically used to replace parts of a track by another track the container level. This can be useful in video to encode a whole movie normally and have some parts with translated text on the video encoded in different tracks. But then it says the overlay picks the first available track, so I don't know how this can be of any use.

Maybe silently replacing parts of a video that have been removed by another part without reencoding, maybe even using a different codec. In this scenario the multiple TrackOverlay and the order to pick them would correspond to different edits of the video file.
You should move the TrackOverlay change in a separate PR as it's not used at all by the TrackLink feature.

For the TrackLink part, I think all the values should be held in a master element. When translating the spec to code, if the generated code has hierarchy (ie not a flat structure) it's easier to look for the master element and then its children, rather than iterating through all the elements of the TrackEntry to find all the TrackLink values.

[rcombs]

Forgot about this PR for (checks notes) almost 4 years, but I've rebased this and addressed all the comments.