[GNC] Posting sales in multiple currencies (2.6.21)

[Matthew Pounsett]

A short aside.. I think there may still be a bug here.

Even though Assets:Accounts Receivable is set to Placeholder, it gets
offered as a choice for posting to when I'm posting an invoice... only when
I try to post to it I get an error that I'm trying to post to a placeholder
account.  It should probably be excluded from the list of choices.

I'll add that to the bugtracker.