event: add recurrence property & has-recurrence function

Recurrence property of the event enables us to add recurrences
to it in the future. It is the first step in that direction.

https://bugzilla.gnome.org/show_bug.cgi?id=782755

src/gcal-event.h

@@ -76,6 +76,8 @@ const gchar*         gcal_event_get_description                  (GcalEvent
 void                 gcal_event_set_description                  (GcalEvent          *self,
                                                                   const gchar        *description);
 
+gboolean             gcal_event_has_recurrence                   (GcalEvent          *self);
+
 gboolean             gcal_event_has_alarms                       (GcalEvent          *self);
 
 GList*               gcal_event_get_alarms                       (GcalEvent          *self);