Add set/getDefaultShape.

This is necessary so Spout can give a block material set in a world a
default shape (such as if all SolidGreen blocks will have a basic 1x1x1
BoxShape).
Signed-off-by: Chris Sanders <zidane@outlook.com>
Switch error blocks to BasicSolids.

Define shape constants in their own classes (this really helps cut down on
object amounts). Make BasicSolid have a default shape.
Signed-off-by: Chris Sanders <zidane@outlook.com>