FCP XML: Support appropriate top-level objects beyond project

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`
1. `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.

Start at the FCP XML adapter's existing top-level export and import handling. Compare the current project-wrapped timeline behavior with the requested mappings for Timeline, SerializableCollection, clip, sequence, bin, and project, then verify that each listed single- and multiple-object case returns or writes the specified top-level form.

Written by the indexing model from the issue text.