Add a default close/dismiss button to all notifications (a new themable part), to avoid the need for application develpers to do that manually. It is considered a best practice to have an explicit dismiss button for notifications.

Add a new theme variant that the developer can use to opt-out of (hide) the default close/dismiss button.

Open questions:

• How do we handle the close/dismiss button label, how is it localized (do we need an i18n property)?
• Should ESC close/dismiss the notification, when the notification itself is focused (see vaadin/vaadin-notification#121)?