Update order_total when order saved

When an order is saved, implementations of 'save' and 'presave' hooks (e.g. uc_taxes) can change the line items and therefore render the order_total property obsolete. In particular, this causes 'uc_payment_condition_order_balance' to fail when applied as a payment method condition during checkout ajax submissions. This is what's behind #1446176: Refresh available payment methods after applying a coupon. We should update the order_total after an order is saved as in the following patch.