Add support for listening on individual notifications

We currently rely on FooListener interfaces to attach to notifications. This has an unfortunate effect that users cannot specify individual notifications they are interested in. This is a similar problem we have faced with Actions vs. RPCs, and the Action invocation model is far superior.

Introduce support for action-like single-notification listener via a dedicated interface, which will allow users a more granular access to notifications.

We will also need to support aggregate listeners (because that is what FooListener does), but unfortunately DOM APIs are not quite ready to offer us that.