Allow users to view/edit UsdCollectionAPI in Outliner

We are developing workflows for our artists that use the UsdCollectionAPI to emulate native Maya features like display layers and selection sets. However, these are collections are currently blind data to the artists - we have to write tools for them to create, view, and edit the contents of these collections.

As a user, I would like UsdCollectionAPI objects and their contents to be presented and editable in the the outliner.

Start by reviewing the existing Maya Outliner integration and the UsdCollectionAPI workflow described in the issue. Done means UsdCollectionAPI objects and their contents are visible and editable in the Outliner, without requiring separate artist tools.

Written by the indexing model from the issue text.