Multi-currency "Post invoice" improvements

- only pop up the exhange rate notification message when an exchange
  rate is effectively asked. Still only pop it up once per post operation.
- ask for conversion rates again if an invoice is reposted on a different date
- only keep one price per foreign currency in the invoice's price list

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23491 57a11ea4-9604-0410-9ed3-97b8803252fd