Crashes when reading OTIO file with SerializableCollection

[jminor]

If Raven is given an OTIO file with a top-level object that isn't a Timeline, it crashes. The most common case of this is an OTIO file with a SerializableCollection containing multiple timelines.

At the very least, it should give the user a helpful error message instead of crashing.

Even better it could present each different type of OTIO object in a reasonable manner. For SerializableCollection, maybe tabs for each item. For other types like MediaReference, Clip, etc. it could just show the JSON and Inspector without the timeline view.

[jminor]

Here's an example of a case where it would be useful to view a SerializableCollection: AcademySoftwareFoundation/OpenTimelineIO#1687