Replace of int's with ArtifactID and fix for 1392

Usage of constants should make it a bit easier to find some code related to catapult/spellbook specifics.

Now it's possible to sell spell scrolls:
http://bugs.vcmi.eu/view.php?id=1392
@alexvins IIRC at least one static code checker (can't remember which one) shouts about "case without break" in such occasions. I'm sure they are smart enough now, but just in case it won't break anything.