Allow for root layer naming on stage creation

[vinRosso]

From AOUSD#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.