Support multiple serializable collections

[vade]

In the latest OTIO meeting (Feb 27th) it was pointed out that multiple top level serializable collections is technically supported in OTIO not just a timeline.

In theory, multiple timelines can be included in a single OTIO file, as could just arrays of clips sans timeline.

Proposal

• Leverage the sidebar to expose any of the top level OTIO serializable collections
• Allow a user to select a collection to be viewed in the main UI (ie if we have 4 timelines and 10 clips) allow each to be selectable.

For timelines, the UI is clear

For a clip, slightly less clear, but we can figure something out.

@jminor is it possible to include in this issue some OTIO files that have multiple top level collections? Are there any other affordances that may need tended to in terms of the types of top level collections?

[vade]

https://github.com/AcademySoftwareFoundation/OpenTimelineIO/wiki/OpenTimelineIO-Application-Integrator's-Guide#root-objects-in-otio-files

[vade]

See https://academysoftwarefdn.slack.com/archives/CMQ9J4BQC/p1751462187550049 as well