API improvements to spell books:

- Allow spells to set which book/scroll items they can appear on
- Allow spell books to set custom GUI textures
- Tweaks to loot functions and wizards to accommodate the changes

src/main/java/electroblob/wizardry/entity/living/EntityWizard.java

@@ -780,6 +780,7 @@ public class EntityWizard extends EntityCreature implements INpc, IMerchant, ISp
 		Tier maxTier = Tier.NOVICE;
 
 		List<Spell> npcSpells = Spell.getSpells(Spell.npcSpells);
+		npcSpells.removeIf(s -> !s.applicableForItem(WizardryItems.spell_book));
 
 		for(int i = 0; i < n; i++){