views/agenda: Make list items unfocusable

Without this, the entire row would become focusable, which is
an undesired functionality for people who navigate with a keyboard.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/807>

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

@@ -37,6 +37,7 @@ template $GcalAgendaView: Box {
         template ListItem {
           activatable: false;
           selectable: false;
+          focusable: false;
 
           child: $GcalAgendaViewDayRow {
             day: bind template.item;