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