3D models as cockpit

[levinli303]

render a 3D model at a fixed orientation/position relative to the observer (in VR version untransformed orientation of the observer) after rendering objects and before rendering HUD.

a possible simple implementation:

1. reuse Body class and provide CelestiaCore::setCockpit(Body *) which creates a copy of the body as the spaceship/cockpit (detach it from its planetarysystem in its copy and make other changes if necessary)
2. in Body definition in ssc, add properties to hold the cockpit position relative to the body and initial orientation
3. updates the global position and orientation per render loop
4. always render it at the end of object rendering, the lighting should be based on the closest stars/planets etc...