Improve bank accounts selection on accounting transactions

Fix small problems displaying bank accounts on accounting transactions and be more strict during validation:
 - bank account must be validated
 - bank account must belong to the section

The latter was a problem for scenarios like this:
 - create transaction for supplier 1 with supplier 1 bank account
 - clone transaction
 - change to supplier 2
the bank account remains as supplier 1 bank account, although it became an invoice from supplier 2.
The transaction could be validated as long as the bank account is not invalidated

See merge request nexedi/erp5!1713