[nemo-qml-plugin-calendar] Notebook management related changes.

- Add calendar uid property to event. Required by qml to select notebook for saved event.
- Fix event::realonly to return notebook readonly status where event is stored.
- Change event:save to require notebook uid. Enables moving event to other notebook.
- Fix missing colorChanged signal emit.
- Add readonly role to notebook model.
- Add localCalendar role to notebook model.