Allow for root layer naming on stage creation

# From [AOUSD#914](https://forum.aousd.org/t/changing-tag-for-anonymous-layer/914)
In USD is possible to assign a tag to the root and session layers when creating a stage in memory, but not to edit the identifier of anonymous layers.

Since the Layer Editor uses the tag to show the anonymous layers names, it would be good to ask the user for an optional name when creating an empty stage or sublayer.

In the example below the two sublayers have been created by code, while the root layer is created in Maya.

Start with the Layer Editor's empty-stage and sublayer creation entry points described in the issue, then compare how root and session layers receive tags when a stage is created in memory. Done means an optional user-provided name is accepted for the root layer when creating an empty stage or sublayer and appears in the Layer Editor.

Written by the indexing model from the issue text.