add vertex color smooth operator.

Since the bug was fixed that made blur brush _not_ blur all verts, there is no way to blur vertex colors after baking.
While this was hidden it was useful especially for baking vertex colors.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56347 954f8c5b-7b00-dc11-b283-0030488c597c