shell_util_normalize_and_casefold: New utility function

Merge the duplicated copies into shell-util.

src/shell-util.h

@@ -21,6 +21,9 @@ void     shell_util_get_transformed_allocation (ClutterActor     *actor,
                                                 ClutterActorBox  *box);
 
 int      shell_util_get_week_start             (void);
+
+char    *shell_util_normalize_and_casefold     (const char       *str);
+
 char    *shell_util_format_date                (const char       *format,
                                                 gint64            time_ms);