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
This may be a stupid question, but what about references? I am guessing references will be returned here? Is that also exported by the alembic extension? What if a reference is turned off? Is it a what you see is what you get type of behaviour? Given some of the future ideas of multiple references, LOD switching etc, would it make sense to *not* export references by default?