[nemo-qml-plugin-calendar] Add new Calendar API class
[nemo-qml-plugin-calendar] Remove Qt 4 support
[nemo-qml-plugin-calendar] Add event deletion methods
[nemo-qml-plugin-calendar] Maintain models across database reload

Also add a EventQuery object to be able to load a single
event, and add remove() APIs to the Calendar API object.
[nemo-qml-plugin-calendar] Wait for completion before populating AgendaModel
[nemo-qml-plugin-calendar] Give access to event exception times