[22673] Use the correct fraction when editing expanded multiple currency transactions.
When a transaction is expanded and trading accounts are in use, the debit
and credit cells contain values in the split's commodity not the transaction's
currency.  For example if you have a transaction involving JPY and USD
currencies and the transaction's currency is JPY, you can only enter whole
dollar amounts in the USD splits.  This fixes this problem.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/branches/2.4@22737 57a11ea4-9604-0410-9ed3-97b8803252fd