Rocket sprites

Built and tested the carryable rockets. Also need the 'dummy' rockets in the distant view. These will be 'furniture' sprites.
After long contemplation, decided that the furniture sprites, though they tend to be static, should also be MOVABLE, so they can be shoved into Limbo, as will happen with the dummy rockets. So probably type is an irrelevant attribute of Sprite, but I'll leave it there for the time being. The other attribute - GETTABLE, however, is significant, as is ACTIVE.