FCP XML: Support appropriate top-level objects beyond project

[reinecke]

Right now the FCP XML adapter writes out timeline objects under a project. The FCP XML spec designates you can actually have project, bin, clip, and sequence objects as top level.

Given the OTIO data model, the best analogues would be to switch on the object type passed to the adapter to write and map as follows:

1. Timeline -> sequence
2. SerializableCollection -> bin

I don't think there is a reasonable analogue for project in otio. Perhaps passing a single otio Clip object could write a top-level clip, however I think it's much more in-line with otio usage to write a SerializableCollection top-level.

For import, I think the behavior should be:

• If the top-tag is a project and there is only one child element, it's object will be returned as the top-level object, otherwise a SerializableCollection will be returned.
• If there are multiple top-level tags under the xmeml tag or a single clip, these will be returned in a SerializableCollection
• A single sequence is returned as a Timeline
• A single bin is returned as a SerializableCollection

Elements of this behavior exist already, but this could be more fleshed out.

[ssteinbach]

What is in the Project but not in a bin or sequence? So far we've resisted having a 'Project' type object

[wrosecrans]

AFAICT, "Project" literally just has Children and a Name: (Where "Children" can just be Clips, Timelines, or Bins)
:
https://developer.apple.com/library/archive/documentation/AppleApplications/Reference/FinalCutPro_XML/Elements/Elements.html#//apple_ref/doc/uid/TP30001156-TPXREF104

It can probably be mostly ignored for OTIO schema sake, and treated as a Collection like a Bin that is at the top/root level. (And any comment/label metadata applied to the top level collection just wouldn't survive being written to xmeml, because the Project isn't supposed to have them.)

[ssteinbach]

Ah, yup. That sounds like a SerializableCollection. Sounds good, thanks!