wrong cash discount calculation of USD invoice [account_invoice_cash_discount]

[Thorsten Vocks (OpenBig.org)]

Use Case is:

- Supplier Invoice 1000 USD + 19% tax
- Cash discount 2% in 10 days
- Payment of a supplier invoice via "Enter Bank Statement" (for USD invoice in a USD bank journal)
- Import of supplier invoice as payment line

Result: Wrong leading sign. Imported supplier invoice would rise up the end balance and it should
be the opposite. I changed this manually to a minus.

Also the deduction of cash discount seems to be with a wrong leading sign, because the value in
the field reconcile is higher and not lower than the value in the field amount.

On details of reconcilation field (F2) all fields under "write off and cash discount are converted
in EUR, for instance total entries, balance (attention: follow up bug of leading sign) or total write-
off.

The discount and the tax move line in this area seems to be shown in USD (not converted to EUR).
I would expect to receive right results also a conversion of 2% cash discount from 1190 USD to
EUR. We expect to receive account move line calculated with the following logic:

Cash discount amount in USD / Currency Rate USD, for instance 20 USD / 1.25 = 16 EUR.
We need also tax correction in EUR for the right results.