Items: Add isGold() function to items.h

This patch adds isGold() function to items.h, which checks if the item's
type is gold.

Source/items.h

@@ -333,6 +333,15 @@ struct Item {
 		}
 	}
 
+	/**
+	 * @brief Checks whether this item is gold.
+	 * @return 'True' in case the item is gold and 'False' otherwise.
+	 */
+	bool isGold() const
+	{
+		return this->_itype == ItemType::Gold;
+	}
+
 	/**
 	 * @brief Checks whether this item is a helm.
 	 * @return 'True' in case the item is a helm and 'False' otherwise.