Added seller interface for preparing shop items from other plugins.

commodity/commodityservice.cpp

@@ -35,3 +35,8 @@ ShopItemPtr CommodityService::shopItem(int itemId)
     CommodityDataPtr item = this->loadById(itemId);
     return qSharedPointerDynamicCast<ShopItem, CommodityData>(item);
 }
+
+ISeller *CommodityService::seller()
+{
+    return NULL;
+}