rigidbody: Code cleanup

Clean up DNA comments.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54797 954f8c5b-7b00-dc11-b283-0030488c597c
rigidbody: Add motor constraint

It's implemented as a separate constraint instead of adding properties
to the existing constraints.
Motors only apply linear and angular impulses and don't limit the
movement of rigid bodies, so it's best to use them in conjunction with
other constraints to limit the degrees of freedom.

Thanks to Markus Kasten (markus111) for the initial patch.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54798 954f8c5b-7b00-dc11-b283-0030488c597c
rigidbody: Fix motion paths calculation being incorrect for rigid bodies

For the simulation to work properly the limited update the motion paths
calculation did wasn't enough so you got different results for for
motion paths than for the actual simulation.
Now do full frame update if rigid body sim is active.

TODO investigate if we can still limit this.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54799 954f8c5b-7b00-dc11-b283-0030488c597c