rigidbody: Avoid unnecessary simulation updates

Now we flag the world for update on frame change and only call
BKE_rigidbody_do_simulation() when needed.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54588 954f8c5b-7b00-dc11-b283-0030488c597c