Begin on inhibition / do not disturb stuff

- Also moe the NotificationPopup item back to the root folder
- Implement tracking of unknown apps, ie. once an app shows a notificaton it will show up
  in the KCM so it can be configured
- Make blacklisting of notification popups (via setting in KCM) work
- Start on inhibition stuff (DBus API proposal sent to XDG mailing list)