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
These are the only types of layers that are exportable so this is following the same pattern as the Mari exporter - I've added some more comments to explain this though.