timeline: Handle multiday events in calculate_changed_events

This patch do not sends ADD_EVENT for events that are multiday and are
part of the old_range when calling calculate_changed_events. This will
prevent multiday duplicate events to appear in the agenda widget when
using the week view.

If the event is a multiday and it was in the previous range, it won't
send the REMOVE_EVENT so multiday events won't dissapear when it's still
relevant.

See https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/898#note_1602391

src/core/gcal-event.h

@@ -136,6 +136,9 @@ void                 gcal_event_get_original_timezones           (GcalEvent
 
 gchar*               gcal_event_format_date                      (GcalEvent          *self);
 
+gboolean             gcal_event_overlaps                         (GcalEvent          *self,
+                                                                  GcalRange          *range);
+
 G_END_DECLS
 
 #endif /* GCAL_EVENT_H */