Art, pass 1

I think I covered most things. Some could still be animated, the second set of level tiles could be tweaked, I kind of want to add a third set of level tiles, all of the polish in the world could be applied to the knight, etc. But this satisfies me for one PR and three tickets…

Fixes #28.
Fixes #66. (Door has a + for in, - for out)
Fixes #89.