Implement a basic ship frame.

[ecdavis]

Ships are distinct from mobiles. Per the design doc, they are pilotable entities. Implement a basic ship frame (and, by extension, simple entity model) and have movement commands control the ship frame rather than the mobile.

[ecdavis]

This is resolved with the introduction of entities. There is no longer a need for a ship frame separate from the mobile, because mobiles are full-fledged entities.