Bug #700804: Add up/down buttons for ordering of transactions in register2.

This changes the ordering but only for txn with the same date
and same number.  The buttons are active only in cases where this is
possible, otherwise the buttons are inactive anyway.

- for reconciled splits, user will be asked
- book-closing txn are ignored as well as frozen splits
- After changing the sort order, update the UI buttons immediately as well

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23061 57a11ea4-9604-0410-9ed3-97b8803252fd