add Action Window with latest informations for Statements

Following discussion at https://discuss.tryton.org/t/statement-view-for-dashboard/2703:

- add an Action Window (suitable for dashboard) with latest informations for Statement: per journal, last statement date and last statement amount
- extent account.statement.journal with 'last_date' and 'last_amount' Funtion fields
- replace code of Statement.on_change_journal() by using journal.last_amount