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 has been in there for ages (at least pre Maya 2010) but it's a plug-in so it needs to be loaded.  The artists will only get errors if they choose to publish the Alembic and the command isn't available though.