rigidbody: Improve add/remove operators

Now all add/remove operators for rigid body objects and constraints
automatically add objects to the appropriate groups and create groups if they
don't exist yet.

This makes handling rigid bodies easier but doesn't take away functionality.
If users want to handle groups manually they just need to create them before
adding any objects.

The previous behaviour was confusing and was even considered to be a bug since
clicking on rigid body in the physics tab seemed to do nothing.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54094 954f8c5b-7b00-dc11-b283-0030488c597c