25841 Mari

Initial release candidate of the Mari engine

- Includes init.py for engine startup used by tk-multi-launchapp
- The Shotgun->Work Area context menu is slightly different to work around a missing Mari API that means sub-menus can't be deleted. The menu is called 'Current Work Area' and the first menu item describes the work area (e.g. 'Asset blah, Texturing')
- Contains utility functions for managing Shotgun metadata on projects, geometry and geometry versions which are then used by various apps/hooks when interacting with Mari objects. Basically it Toolkitifies a bunch of Mari stuff!