[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 and rename isDefault role to default.