calendar: Improve week start handling

Add a helper function (mostly copied from gtkcalendar.c) for getting
the first week day for the current locale, using nl_langinfo if
available and falling back to the GTK+ gettext fallback otherwise.

Use that function in the calendar, so that the LC_TIME setting is
used if possible.

https://bugzilla.gnome.org/show_bug.cgi?id=649078

src/shell-util.h

@@ -20,6 +20,7 @@ void     shell_util_set_hidden_from_pick       (ClutterActor     *actor,
 void     shell_util_get_transformed_allocation (ClutterActor     *actor,
                                                 ClutterActorBox  *box);
 
+int      shell_util_get_week_start             (void);
 char    *shell_util_format_date                (const char       *format,
                                                 gint64            time_ms);