implement setContactResponse(boolean) for PhysicsRigidBody (issue #964)

[stephengold]

Implementation for PhysicsCharacter will be a separate PR.

[riccardobl]

I know i was the one who suggested the name for this method, but on second thought i think it would be better (more descriptive) to invert the logic and call it setIgnoreContactResponse or setNoContactResponse so that it mirrors the flag name.

Also i think hard coded flags should be avoided, by either doing this or by making set(No|Ignore)ContactResponse native.

[stephengold]

Riccardo: Thanks for reviewing this PR. I appreciate your help.

While I see value in being consistent with the flag name, I'm reluctant to alter the method names because of the extra mental effort to interpret "set ignore" or "set not" as compared with "set".

I agree that hardcoded flag values are undesirable. That was lazy of me. I'll address it before integrating this PR.

I've also decided that isContactResponse() should be a PhysicsCollisionObject method rather than just a PhysicsRigidBody one, so I need to update this PR anyway. (The setter will remain specific to PhysicsRigidBody.)

[stephengold]

In the process of testing load/save/clone, I came across some unrelated bugs in jme3-bullet, but I'm satisfied that load/save/clone work for contact response. Once I've documented those issues, I'll look at PhysicsCharacter.