Make featured sketches bundled with the app (#391)

* Replace Poly Featured Sketches with sketches bundled with the app.

* Move featured sketches in to persistent data

* More featured sketches from Icosa. Also removed the old Poly/json ones.

Rename sketches to have artist name, and remove those we don't have permission for yet

* Present tilt files that have ' by ' in the name as being <title> <author>

* Switch out some larger sketches for smaller ones that I now have permission to use.

Author: TimAidley