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
Does it make sense to have an else here? Just double checking since I don't know much about mari? :) If there is an expectation that layers are being exported and certain layer types are omitted, maybe a warning should be emitted saying "skipping layer x - unuspported type" or something...