Protect NemoCalendarWorker::setDefaultNotebook() against null
[nemo-qml-plugin-calendar] Introduce notebook query class

Fetches information about notebook based on uid.
[nemo-qml-plugin-calendar] Add property for default notebook uid in calendar api
Remove change signals that happen before changes
Merge pull request #33 from pvuorela/notebookquery

Added notebook query and interfaces for default notebook