Allow for discounts, VAT and other price components to be included in the refund

Currently refunds are quite limited and only take the total sum for a line item and divide it by the quantity to get the unit price. This creates discrepancies in for example VAT reports since there is no way to subtract a refund correctly. I think that I've found a relatively simple solution to this issue that also correctly reports product discounts and other price components. Please see patch for details.