Aqbanking online transfer: Set the date in the gnucash txn dialog to non-sensitive.

Users regularly expect that date to be the online job execution date,
but that's not what is implemented. So we prevent editing here at all.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@22784 57a11ea4-9604-0410-9ed3-97b8803252fd