[FIX] account_voucher: validation of refund in multicurrency

Fixing several issues for refunds in multicurrency mode that prevented the moves to be balanced.
The amount_currency needs to use the absolute value, as a refund will have a negative amount.
The sign for currency_rate_difference needs to use the line instead of the voucher as they can have different value. e.g. a voucher of 1000$ with invoice of 1200$ and refund of 200$ will have two lines and their currency_rate_difference should have different signs.
Avoids doubling the value in foreign_currency_diff
Fixes #1490, opw 607118 & 611580