Introduce the new calendar monitoring API

It is, as expected, heavily inspired by the ECalDataModel code,
except it uses GDateTime, is timezone aware, and can handle all
day events correctly.

src/core/gcal-event.h

@@ -131,6 +131,10 @@ void                 gcal_event_get_original_timezones           (GcalEvent
 
 gchar*               gcal_event_format_date                      (GcalEvent          *self);
 
+gboolean             gcal_event_is_within_range                  (GcalEvent          *self,
+                                                                  GDateTime          *range_start,
+                                                                  GDateTime          *range_end);
+
 G_END_DECLS
 
 #endif /* GCAL_EVENT_H */