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