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
So something like that would work?
  bool CArtHandler::isTradableArtifact(ArtifactID id) const
  {
  	switch (id)
  	{
  	case ArtifactID::SPELLBOOK:
  	case ArtifactID::GRAIL:
  	case ArtifactID::CATAPULT:
  	case ArtifactID::BALLISTA:
  	case ArtifactID::AMMO_CART:
  	case ArtifactID::FIRST_AID_TENT:
  		return false;
  		break;
  	default:
  		return true;
  		break;
  	}
  }