Remove separate ThrustDiagram

Currently, RV uses a separate `FormDiagram` and `ThrustDiagram` for the 2D and 3D force layouts. The `ThrustDiagram` is superfluous and should be removed.

For this to work, among a few other things, we need to redefine the scene object associated with the `FormDiagram` to provide draw methods for both the 2D and 3D representations, and call those methods from the main draw method as needed.
