Add undo / redo to Edit-as-Maya workflows.

[pierrebai-adsk]

Create commands:

• Add mayaUsdEditAsMaya command for edit as Maya.
• Add mayaUsdDuplicate command for duplicate as Maya and duplicate to USD.
• Add mayaUsdDiscardEdits command for discarding edits.
• Add mayaUsdMergeToUsd command to merge back to USD.
• This makes it easy to script and support undo/redo.
• Remove the old mel script for pull/push/discards/duplicate, replace them with these commands.
• Add unit tests for all new commands.

Create system to record undo items:

• Add OpUndoInfo to record all undoable steps for each commands.
• Add an instance to UsdUndoManager singleton to record undo items from anywhere.
• Add OpUndoItem to record a single undoable sub-operation step.
• Add OpUndoInfoMuting to muting recording undo items in block of code that handles undo differently.
• Add OpUndoInfoRecorder to automatically record all undo items in a block of scoped code.

Provide undo items for many common scenarios:

• Implement OpUndoItem for USD undo block (UsdUndoBlock / UsdUndoItem).
• Implement OpUndoItem for MDagModifier.
• Implement OpUndoItem for MDGModifier.
• Implement OpUndoItem for selection.
• Implement OpUndoItem for locking nodes.
• Implement OpUndoItem for pair of Python do/undo scripts.
• Implement OpUndoItem for any generic pair of do/undo C++ functions.

Various code adjustments:

• Remove the cached pull root from PrimUpdaterManager since it was interfering with undo/redo.
• Retrieving it for each command is not costly and avoid needing us to clear it.
• Use undo info muting in read job (import) class since it implements its own undo / redo.