CameraDisplay added

This is the first part bit of effort in making a camera that tracks the
player/background/enemies and moves everything relative to a particular
object. It uses a stack of DrawableObjects to determine which object to
track next when one is removed.

This class will still need to have some sort of velocity applied to it
when moving. It should not be instantaneous as it is now, but for the
moment it will move any object to the center of the screen and track it.