Added movable mixin, fire cell, changed sprite/cell properties

Sprite and cell properties are now class attributes, which cuts down on
syntax. Shouldn't cause any unexpected behaviour as long as these
contain immutable values.

Added the fire cell and the movable mixin. Movable items glide over fire
cells like in Rescue Rover. Rover level 1 added (partially) to
demonstrate this.
Added shooter and laser sprites

And updated Rover level 1.