EMSUSD-1098 Fixed saving vs locked layers

[pierrebai-adsk]

Also EMSUSD-1097, EMSUSD-1099 and EMSUSD-1100

EMSUSD-1097 Refactor anonymous layer saving. There was code that was nearly identical between the Layer Editor item saveAnonymousLayer function, the saveSubLayer function and the generic saveAnonymousLayer function in utilSerialization.h. Refactor the layer editor item to use the generic one instead, avoiding having to fix problems in two places.

• Remove the saveSubLayer function that is now entirely redundant.
• Refactor the LayerTreeItem function to use the generic saveAnonymousLayer function.
• All the code left in the LayerTreeItem is now specific the the layer editor or its session state.
• Add a FileBackup helper class in the utilFileSystem to backup a file and possibly restore it if the new file is not commited as valid.
• Add a parameter to the generic saveAnonymousLayer function to return an optional error message.
• Improve error messages when saving
• Prevent saving system-locked layers.
• Add helper functions to transfer the muted and locked status of anonymous layers to their new layers.

EMSUSD-1100 preserve the locked and muted status of layer when saved. When transitioning from anonymous to non-anonymous, update the muted and locked status of the layers.

EMSUSD-1098 don't allow saving system-locked layer Prevent saving when double-clicking on a system-locked layer.

EMSUSD-1099 fix parent layer when converting anonynous layers. The parent layer that was passed to saveAnonymousLayer when converting anonymous layers into saved layers was incorrect. It was passing the parent of the parent instead. This lead to the stage root layer getting replaced by its oen sub-layer!

Add unit tests and Python wrappers

• Add Python wrappers to lock, unlock and verify lock on layers.
• Write unit tests to verify the behavior of locked layers.
• Improve the tests for muted layers.

[pierrebai-adsk]

PF failure was files being left in tmp folder in one Windows build.