Store CreatureIndex on Creature, includes helper function to get a creatures party from a battle.

Signed-off-by: Deukhoofd <Deukhoofd@gmail.com>

src/Battling/Models/CreatureParty.hpp

@@ -63,6 +63,9 @@ namespace CreatureLib::Battling {
                 }
             }
         }
+        inline bool HasCreature(ArbUt::BorrowedPtr<Creature> creature) const noexcept {
+            return _party.Contains(creature);
+        }
 
         virtual CreatureParty* Clone() const {
             auto party = new CreatureParty(_party.Count());