USD Layer Editor should allow for layer renaming

[dgovil]

As our artists are starting to use the UFE workflows, a common request has been the desire to rename layers after they've been created. If they make a mistake during the naming process they need to go manually chase down everything at this time to reflow it.

If the layer editor could have the ability to rename layers, it would allow for artists to more easily change things like they would with any other file based workflow.

[MiraBlack]

Our production needs in renaming too. It's hard to name files as needed in-place in our pipeline, so this feature will be very usefull.