Cycles: ray visibility panel is now also available for the world, works same as
meshes and lamps. The light path node already made this possible but it's a bit
faster to render this way and convenient.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57363 954f8c5b-7b00-dc11-b283-0030488c597c