BulletSim: Add IsSelected attribute to physical objects. Have vehicles check to see if physical before trying to step. Replace vehicle gravity application. Previously relying on Bullet to apply gravity but since vehicles over-ride the velocity calculation, gravity never had a chance to accelerate the body down. Added AddForceImpulse as well as AddForce for those who need to apply immediate velocity updates. Use the impulse to apply the linear motion.

OpenSim/Region/Physics/BulletSPlugin/BSPhysObject.cs

@@ -135,6 +135,7 @@ public abstract class BSPhysObject : PhysicsActor
     public virtual OMV.Vector3 Scale { get; set; }
     public abstract bool IsSolid { get; }
     public abstract bool IsStatic { get; }
+    public abstract bool IsSelected { get; }
 
     // Materialness
     public MaterialAttributes.Material Material { get; private set; }