rnd-20070926-2-src

* added new custom artwork setup option "auto-override non-CE sets" for
  automatic artwork override that is only used for level sets without
  custom element artwork (as it does not make much sense to override
  any artwork that redefines custom element artwork for sets using CEs)
* fixed default artwork for "special" R'n'D versions always using the
  "classic" artwork as the base if base artwork is not explicitly
  defined in "levelinfo.conf", regardless of different default artwork
  used by the special R'n'D version -- this is needed because any such
  custom artwork is designed using the "classic" artwork definitions as
  the base (including menu definitions and screen positions etc., which
  would otherwise be taken from the different special default artwork)