25946 Mari support

- Added Alembic Cache publish to Maya secondary publish hooks
- Added Mari publish hooks and functionality to export texture channels & layers
- Fixed resizing issue in items list
- Unrecognised items returned by the scan-scene hook are now ignored - previously they would raise an exception
As this is primarily for the Mari workflow atm, I think this should be ok as it's dealing with assets that typically wouldn't contain referenced geometry.  I'm also keen to keep this hook simple if possible!  I agree that this would be a good topic for general workflow design though as we move forward...