Add controllable vehicles

[rjkilpatrick]

Is your feature request related to a problem? Please describe.
Currently vehicles are just MeshInstances, which are static and do not interact with the games physics.

Describe the solution you'd like
An inheritance hierarchy for vehicles that fly i.e. have 6 degrees of freedom (all 6 may be overkill)
Vehicles to be "enterable", i.e. the player becomes part of (a child of) the vehicle and can control it.