Implement notifications

[matzipan]

When there's more than two new emails do not send a notification for each one

[matzipan]

let notification = gio::Notification::new(&"message");
                        notification.set_body(Some(&"New message"));
                        notification.set_priority(gio::NotificationPriority::Normal);
                        gtk_application_clone.send_notification(Some(&"test"), &notification);