Refactor of SpaceShip.

Partial refactoring of SpaceShip, mainly of the update method that was
quite long and hard to read/figure out what was going on.  I believe the
modifications make it easier to follow what is being done.

Also, removed a couple of magic numbers, but some work remains to be
done.

Attention should be paid to the way InputHandler is saved in the super
class but is used only by the SpaceShip.

Finally, the current update method has a big problem of temporal
coupling due to the way cx/cy are saved within the super class.
Changing them to a Point2D would permit the updateShip... methods to
avoid this dirty coupling.  This remains to be done.