[notification] Add close/dismiss button

[jouni]

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)?

[rolfsmeds]

• Having a dismiss button rendered by default would be a breaking change, and cause issues in existing applications, many of which implement their own dimiss buttons.

• A dedicated boolean property for having a dismiss button might be a cleaner approach.

[jouni]

Right, this could be opt-in initially, and marked to be changed to opt-out in the next major version.

[mvysny]

Passing in the '0' duration will cause the notification to stay displayed; since there's no way for the user to close the notification without the close button, the notification pretty much stays around indefinitely (unless a close button is added manually by the programmer).

Currently, the only way to make a persistent notification with a close button is this very very chatty code:

final Notification notification = new Notification(new Span("An application error happened"));
notification.setDuration(0);
notification.setPosition(Notification.Position.TOP_CENTER);
notification.addThemeVariants(NotificationVariant.LUMO_ERROR, NotificationVariant.LUMO_PRIMARY);
final Button closeButton = new Button(new Icon(VaadinIcon.CLOSE));
closeButton.addThemeVariants(ButtonVariant.LUMO_ICON, ButtonVariant.LUMO_TERTIARY);
closeButton.addClickListener(e -> notification.close());
notification.add(closeButton);
notification.show();