to get ready for...
Start creation of BulletAPITemplate. This defines the abstract interface
functions. Following commits will move over to the new interface.
This will enable switching between the managed and unmanaged version of
Bullet.
While fixing the above, add methods to physical body and shape pointer
wrapper so routines won't have to know that IntPtr.Zero means no
physical instance.
Fix problem with physical linksets failing after a few sits and
unsits by properly restoring child prom positions for compound
linksets after multiple selection and deselections.
