Final touches

- Rename some of the classes:
  NotificationServer -> Server (there's namespaces, you know)
  NotificationModel -> NotificationsModel (so it's plural like JobsModel)
- Introduce NotificationGroupingCollapse
- Wire up PulseAudio-qt for eventual notification silence in dnd mode
- Touch up notification looks
  - Add indentation and "line" for grouped plasmoids
  - Rethink "show more" button to be at the end
  - Fix buttons overlapping
  - Remove NotificationDelegate item and do those few adjustments in FullRepresentation
- Cleanup job details, handle when processed > total
- Use States {} more
- Show low urgency popups by default but don't add them to history