[alarms] Support for read-only calendar alarms, add type property

If the Maemo::Timed::Event event contains the attributes startDate and endDate which are valid ISO 8601 extended format dates, then interpret the event as a calendar alarm. Calendar alarms are read only, they cannot be added through this plugin, only read. 

The startDate QML property indicates when the event starts, endDate indicates when the event ends.

The allDay QML property simply indicates whether the startDate and endDate times are both 00:00, in which case the calendar alarm is interpreted as an all day event.

The type QML property is one of the enumeration values Alarm::Calendar, Alarm::Countdown, and Alarm::Clock,
the property indicates the type of the alarm.