Core/Units: reworked creature power regeneration handling

* players and creatures now share the same power regeneration regeneration function
* UpdatePowerRegeneration is now virtual and handles creature and player specific formulas individually
* handle update field flag UF_FLAG_UNIT_ALL to handle UNIT_FIELD_POWER_REGEN_FLAT_MODIFIER and UNIT_FIELD_POWER_REGEN_INTERRUPTED_FLAT_MODIFIER for creatures as well

src/server/game/Entities/Object/Object.cpp

@@ -748,6 +748,9 @@ uint32 Object::GetUpdateFieldData(Player const* target, uint32*& flags) const
 
             if (plr && plr->IsInSameRaidWith(target))
                 visibleFlag |= UF_FLAG_PARTY_MEMBER;
+
+            if (IsCreature())
+                visibleFlag |= UF_FLAG_UNIT_ALL;
             break;
         }
         case TYPEID_GAMEOBJECT: