Valid order statuses improvements

This PR addresses an issue where it was possible to filter the valid order statuses for payment on my-orders.php, checkout and in order class, but not in the actual pay_action in `wc-form-handler.php`.

Also, it introduces the `woocommerce_valid_order_statuses_for_payment_complete` filter which further allows developers to decide which order statuses are valid for completing a payment.