Fix the crash that occurs when duplicating multiple invoices from Find results.
The biggest problem was that it was adding an invoice's entries to both the old
and the new invoice when duplicating the invoice.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23070 57a11ea4-9604-0410-9ed3-97b8803252fd
Make it possible to post multiple invoices from the find dialog.
GnuCash was crashing if you searched for all unposted invoices, selected two or
more of them, and posted them.  This was because the list of invoices to be posted
changed while it was being processed.  As part of fixing this the prompt for post date,
due date, etc. will only be given once, not once per invoice.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23071 57a11ea4-9604-0410-9ed3-97b8803252fd