reorganized inheritance (Spaceship now extends SpaceObject. Spaceship, Projectile, Upgrade are all siblings.) Upgrade infrastructure is now correctly
implemented in such a way that is also extensible at all levels (effect, image, etc.) Upgrade collision detection finalized.

Upgrade effect for upgradeType != 0 on Player still needs to be implemented.
Replay functionality is broken and has been disabled.

Drawable interface has been added, but not implemented. Consider removal.