Save couponAmount and productAmount in newly created Order

In order for the couponAmount and productAmount to be correctly stored
during th CartOrderTransformer::createOrderFromCart() process, we must
ensure that a correct amount is initialized in the Cart object when the
cart.onload event is triggered.
@xphere that's right, the correct place for this would be some event listener in the `CartCoupon` component that listens to the `ElcodiCartEvents::ORDER_ONCREATED` event. If there are no other side effects (there shouldn't) this would be a quick change. Trying it :+1: