Move Steal_Hero mechanics out of hardcode and into creature properties.

[PlebSupporter]

We talked about this a while back but currently steal_hero mechanics are hardcoded to, I believe, creature slot position in the creature.cfg. It would be a big enhancement if this could be changed to two different creature properties, one for creatures that can't be stolen (currently Knight and Avatar, with a name like LOYAL) and one for creatures that spawn when no other options are available (currently Archer and Thief; MERCENARY).

With this change, mapmakers can easily define via .cfg or script via SET_CREATURE_PROPERTY which creatures can and can't be chosen, and also better facilitates boss-type creatures in maps.