This is a major rework of the Agenda view. It splits the view into
GcalAgendaViewDay, which represent a single day with events; and
GcalAgendaViewDayRow, which is the actual widget with a title label,
and a listbox with event widgets.
The Agenda view itself merely manages a list of GcalAgendaViewDays,
and updates them with the events model, and active dates.
This allows us to get rid of the custom listbox row, and the confusing
checks for event widgets, and all the bugs the came with it. It also
allows for removing the internal GcalRangeTree, and delegate all
event management to list models.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/742>
Georges Basile Stavracas Neto