Remove Generic* classes

This removes GenericShip, -WorldObject, -Gun, -Bullet, -Rocket, -RocketLauncher.

They were originally introduced for a maximum of flexibility, but in the end lead to a lot of codeduplication. The virtual getters (cooldownTime(), ...) were replaced by normal getters and setters in the base class. When a specialization wants to change them dynamically, it has to do so in the update() method.