Patch from Mike:

 - remove the backtracking `Hard' level, on the grounds that it was
   incredibly slow and not really usable.
 - introduce an `Easy' difficulty level below the standard one; many
   people seem to find this puzzle unusually hard, so an easy level
   is particularly helpful.
 - highlight unfulfillable clue squares (but not yet any other types
   of obvious error).

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6299 cda61777-01e9-0310-a592-d414129be87e