event-widget: add helper API

Added GcalEventWidget::is_multiday to check if an event span more than
one day.
Added GcalEventWidget::compare_by_length which compares two event
widgets according to the duration of the events they represent.

src/gcal-event-widget.h

@@ -95,6 +95,8 @@ void          gcal_event_widget_set_all_day                (GcalEventWidget    *
 
 gboolean     gcal_event_widget_get_all_day                 (GcalEventWidget    *event);
 
+gboolean     gcal_event_widget_is_multiday                 (GcalEventWidget    *event);
+
 void         gcal_event_widget_set_has_reminders           (GcalEventWidget    *event,
                                                             gboolean            has_reminders);
 
@@ -105,6 +107,9 @@ GcalEventData* gcal_event_widget_get_data                  (GcalEventWidget    *
 gboolean     gcal_event_widget_equal                       (GcalEventWidget    *widget1,
                                                             GcalEventWidget    *widget2);
 
+gint         gcal_event_widget_compare_by_length           (GcalEventWidget    *widget1,
+                                                            GcalEventWidget    *widget2);
+
 G_END_DECLS
 
 #endif /* __GCAL_EVENT_WIDGET_H__ */