USD (sub)layer rollup

**Is your feature request related to a problem? Please describe.**
When I use the layer editor setup, it's easy to make a number of sublayers to build stacks of work or even whole 'variations' of a sort (mute/unmute etc) and see the final composition of my edits.

**Describe the solution you'd like**
In the layer editor I'd like to be able to pick a layer and roll up the edits in its sublayers (or other merge-like behaviour) so that I'm able to jam away with sublayers as a personal organisational system and discard them, while retaining my edits once I'm done.

**Describe alternatives you've considered**
Nothing yet - I don't know if there's really a nice way to do this?

**Additional context**
(feature request originally from an internal discussion - bubbling this up to git for visibility)

Start by reading the layer editor implementation and how it handles sublayers, edits, and mute/unmute state. Determine whether a rollup or merge operation can preserve the composed result while allowing the sublayers to be discarded. Done means a selected layer can retain its sublayer edits after those sublayers are removed.

Written by the indexing model from the issue text.