utils: Use ISO weekday numbering for 'get_first_weekday' function

Change the 'get_first_weekday' function to number the weekdays
Monday(1)-Sunday(7) like ISO does. Change the function name to reflect
that. Add tests for different locales, although the tests depend on
system-installed locales.

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 ();
+  self->week_start = gcal_util_get_first_weekday_iso ();
 
   gtk_widget_init_template (GTK_WIDGET (self));
 

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

@@ -194,7 +194,7 @@ allocate_overflow_popover (GcalMonthView *self,
 static inline void
 update_weekday_labels (GcalMonthView *self)
 {
-  const gint first_weekday = gcal_util_get_first_weekday ();
+  const gint first_weekday = gcal_util_get_first_weekday_iso ();
 
   for (gint i = 0; i < GCAL_N_WEEKDAYS; i++)
     {

src/gui/views/gcal-week-header.c

@@ -100,8 +100,6 @@ struct _GcalWeekHeader
   GtkWidget          *overflow_label[GCAL_N_WEEKDAYS];
   WeekdayHeader       weekday_header[GCAL_N_WEEKDAYS];
 
-  gint                first_weekday;
-
   /*
    * Used for checking if the header is in collapsed state or expand state
    * false is collapse state true is expand state
@@ -1762,8 +1760,6 @@ gcal_week_header_init (GcalWeekHeader *self)
   gtk_widget_set_visible (self->dnd.widget, FALSE);
   gtk_widget_set_parent (self->dnd.widget, GTK_WIDGET (self));
 
-  self->first_weekday = gcal_util_get_first_weekday ();
-
   gtk_widget_init_template (GTK_WIDGET (self));
 
   for (i = 0; i < GCAL_N_WEEKDAYS; i++)