USD Import: support collection instancing

[kowalskima]

This is work in progress. I'm in the process of porting the following feature from branch usd-importer-T81257 to branch usd-importer-T81257-merge:

Provide experimental "instancing" option to import USD scenegraph instances as Blender collection instances. (This will handle only "instanceable" USD primitives, and not point-instancers.)

I will update this issue when I commit this work.

[kowalskima]

I've committed the new experimental Instancing feature to branch usd-importer-T81257-merge. See the attached image of editing a prototype.

Among the remaining issues:

• It would be better if the "prototypes" folder that contains the instance collections were initially closed, to reduce the clutter in the outliner.
• The instance collection names currently correspond to the names of the masters (e.g., /__Master_24). This isn't intuitive and might vary between runs, so the naming scheme should be improved.
• Need to make sure instance collections are properly handled on export as well.

[kowalskima]

Link to commit: https://github.com/tangent-opensource/blender/commit/8b9275d4330d6721e46f02dc0e0f41c585f2807a