proposed invoices in "Select invoices..." wizard [account_payment_discount_extension]

[Thomas Dreiling]

This did not work anywhere near what was expected. The attached patch fixes it.
The change to the search domain is needed so invoices already on a payment order do not get proposed again on consecutive calls of the selection wizard (or later payment orders, even).
The second change is needed for a) unpaid invoices which were due before, which would be filtered out by your if-clause and b) discountable invoices which would lose their discountability before the search_due_date.