EMSUSD-189 detect invalid use of undo item

[pierrebai-adsk]

The OpUndoItem must eventually be kept in a OpUndoList or deleted. This is usually done by using either a OpUndoItemRecorder (to keep them) or OpUndoItemMuting (to delete them). Failure to do so would leak undo items and is a symptom of a programming error where no recorder was created when they should.

Add code to detect such errors in non-release builds. Fixed problems found by the validator.

Adjust the CMake project:

• Add a CMake flag to validate undo item called CMAKE_WANT_VALIDATE_UNDO_ITEM.
• It is on by default in non-release builds.
• It is off by default in release builds to avoid emitting errors that users can't do anything about.
• Add a WANT_VALIDATE_UNDO_ITEM C++ symbol when enabled.

Add the validator:

• Add OpUndoItemValidator class to do the validation.
• Add details about orphaned undo items when the validator fails.
• Add return value to the validator in case calling code want to know the result.
• Don't trigger validation errors when an explicit undo item list is used instead of the global one.
• Print a stack trace of where the validation error are found to more easily find the root cause.

Update related undo item classes:

• Make the recorder and muting classes derive from the validator.
• Make muting code non-inlined.
• Add utility functions to the undo item list class to enable validation and testing.
• Add Python wrapper for the new functionality.

Unit tests and fixes for problems found during testing:

• Add a unit test to validate that the OpUndoItemList works.
• The unit test also indirectly exercises the validator in non-release builds.
• Fix the Python wrapper to not lose the undo items and allow undo and redo.
• Fix the prim updater manager proxy-changed callback to not leave orphaned undo items.
• Add undo item muting in multiple callbacks.
• Correctly mute undo item during the undo and redo of function-based items.
• Fix all tests that did not properly use a OpUndoItemList to capture the undo item of functions. In particular, the edit-as-Maya, merge-to-USD and duplicate functions.

[pierrebai-adsk]

The only PF failure is due to the known random failures in an export test failing to rename a temporary file.

[seando-adsk]

Please look at your option() in CMakeLists.txt

[pierrebai-adsk]

The only PF failure is one known random export failure we've been having recently.