gui: Use the new GcalContext API to get week start day once

This gets the globally set start-of-week setting on application startup.
Making the views redraw themselves on setting change while running will
be a later step.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/822>

src/gui/common/gcal-date-chooser.c

@@ -1083,7 +1083,7 @@ gcal_date_chooser_init (GcalDateChooser *self)
   self->date = g_date_time_new_now_local ();
   g_date_time_get_ymd (self->date, &self->this_year, NULL, NULL);
 
-  self->week_start = gcal_util_get_first_weekday_iso ();
+  self->week_start = gcal_context_get_week_start_day (context);
 
   gtk_widget_init_template (GTK_WIDGET (self));