GcalAgendaViewItem is an auxiliary object that will be used instead of
the raw events by GcalAgendaViewDay in a later commit to construct the
event widgets inside the main ListView of the agenda view as opposed to
a creating them inside of a `GtkListBox` inside each row of the
ListView.
Holding a reference to the event widget will be necessary in later
commits, so when a ListView row is activated, we can query its model of
GcalAgendaViewItem's with the given activated position, and pass the
correct item's event widget to `gcal_view_event_activated ()`
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/809>
