Add a way to signify two (or more) clips are linked/grouped/associated

Often, in editing softwares, different clips are grouped/associated. For example, an audio clip and a video one. Or two video clips, or three...
It could be great to have some information to signify it.

Start by reviewing OpenTimelineIO's existing clip and timeline model, since the issue names no file, test, or entry point. Clarify how linked or grouped clips should be represented for audio and video examples, then define the expected interchange and API behavior before implementation; done means the relationship is representable for two or more clips.

Written by the indexing model from the issue text.