Core/Cleanup: remove C-style cast to (Bag*):

* add ToBag() methods for Item class;
* add GetBagByPos() method for Player class.

src/server/game/Guilds/Guild.cpp

@@ -801,7 +801,7 @@ bool Guild::PlayerMoveItemData::InitItem()
     if (m_pItem)
     {
         // Anti-WPE protection. Do not move non-empty bags to bank.
-        if (m_pItem->IsBag() && !((Bag*)m_pItem)->IsEmpty())
+        if (m_pItem->IsNotEmptyBag())
         {
             m_pPlayer->SendEquipError(EQUIP_ERR_CAN_ONLY_DO_WITH_EMPTY_BAGS, m_pItem);
             m_pItem = NULL;