Introduced BoxBody, CircleBody, PolygonBody, etc. helpers to examples

These make notation slightly shorter in the common case where one
fixture suffices for a body.

Only used in the 'demolition' example for now.
Introduced ImageBoxBody

Not exactly sure yet whether this convenience class is worth it. The
use-case may be common but is also very limited.
Made Body.fixtures the default property

This wasn't possible before when Body was a subclass of Item.
It avoids having to write "fixtures: [...]"