Loading stage layers under new parent xform

Some assets might be exported out without roots, when loading a stage sublayers, all objects from all usd layers are merged down together under the stage, while it's not possible to create xform and do make it parent to the loaded usd sublayer prims.

So I'm suggesting adding an option/checkbox withing the "Load sublayers" dialog beside each sublayer to allow loading it under a newly created xform which gets saved with the stage including the transform edits applied on it.

Start by locating the "Load sublayers" dialog and the stage-loading path it invokes. The issue describes adding a per-sublayer option that creates a parent xform, preserves it with the stage, and applies later transform edits; verify the behavior with sublayers exported without roots.

Written by the indexing model from the issue text.