[FEATURE] submitted amount return in payment object

- If the payment is a partial payment, use the `DeliveredAmount` from the `tx` to fill the `destination_amount` and `source_amount` fields. The original proposed amounts are then displayed in `source_amount_submitted` and `desination_amount_submitted`
Merge pull request #172 from geertweening/partial_payment

[FEATURE] submitted amount return in payment object