Begin settings stuff and further cleanup

- Introduces a Settings item that can be used to query and write settings
  Used from both the applet and the KCM and in the future Task Manager and others
- Wire up delegate to make FullRepresentation somewhat work
- Fix text selection in NotificationItem (workaround QQC1 ScrollView bug)
- Make model writable where it makes sense (rather than having a dedicated setter)
- Let JobsModel also notice jobs that were present at the time of its creation