[nemo-qml-plugin-calendar] Emit updated() on AgendaModel refresh

The updated() signal is emitted when AgendaModel model data has
been updated either as a response to changes in startDate and/or
endDate, or changes in the data base.

The updated() signal does not imply that the AgendaModel has been
changed, only that the contents of the model are up to date.
Merge pull request #36 from pgerdt/agendamodel-update

[nemo-qml-plugin-calendar] Emit updated() on AgendaModel refresh