Move the commands that implement the menu items in their own file as reusable
UFE commands.

- Fix undo of add reference and payload that were adding a delete to the prim
  lists instead of reverting the addition.
- Wrap all move commands in Python
- Add unit test for all commands that were moved into their own file.
- Chang all commands to use UsdUndoableItem and UsdUndoBlock to ensure perfect
  undo.

…dufe

- Fix copyright years,
- Fix header inclusion guards to not use pragma once.
- Move implementation of a function to its own file.
- Put the common text filtering test helper in the usdUtils file.
- Modify tests to use the common function.

In UFE v2, the insert-child command is declared in the hiearchy.h header.