Expanding the track-info extension to include a track index.

[roelofvkr]

I would like to propose to extend the track-info extension to include a track index (or some other kind of representation of the sorting within the host). Both VST3 and AAX support this and I don't see a reason as to why CLAP wouldn't.

In practice this would probably be quite simple and sufficiently usable in the form of an int32_t with a value and a CLAP_TRACK_INFO_HAS_TRACK_INDEX flag.