guis/views: Introduce GcalAgendaViewItem

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>

src/gui/views/gcal-agenda-view.c

@@ -21,6 +21,7 @@
 #include "gcal-agenda-view.h"
 #include "gcal-agenda-view-day.h"
 #include "gcal-agenda-view-day-row.h"
+#include "gcal-agenda-view-item.h"
 #include "gcal-debug.h"
 #include "gcal-enums.h"
 #include "gcal-event-widget.h"
@@ -422,6 +423,7 @@ gcal_agenda_view_class_init (GcalAgendaViewClass *klass)
   GObjectClass *object_class = G_OBJECT_CLASS (klass);
   GtkWidgetClass *widget_class = GTK_WIDGET_CLASS (klass);
 
+  g_type_ensure (GCAL_TYPE_AGENDA_VIEW_ITEM);
   g_type_ensure (GCAL_TYPE_AGENDA_VIEW_DAY_ROW);
 
   object_class->dispose = gcal_agenda_view_dispose;