notification: Add support for getting actions activation token

Notification actions can now be activated with an activation token containing platform data. So update the notification-spec to include latest version of the specification, and expose the activation token during an action activation.

Not to change the API, by modifying the type of NotifyActionCallback it's just better to provide a function to fetch the activation token during activation only, so that this can be retro-compatible.

See https://gitlab.freedesktop.org/xdg/xdg-specs/-/commit/b9a470004d