Work around Spidermonkey problem with Unicode date formats

Monkey-patch Date.prototype.toLocaleFormat() with a version that uses
g_date_time_format() since the Spidermonkey built-in can't handle
format strings with Unicode characters.

https://bugzilla.gnome.org/show_bug.cgi?id=643350

src/shell-util.h

@@ -13,6 +13,9 @@ GIcon *shell_util_get_icon_for_uri (const char *text_uri);
 GIcon *shell_util_icon_from_string (const char *string, GError **error);
 void   shell_util_set_hidden_from_pick (ClutterActor *actor, gboolean hidden);
 
+char *shell_util_format_date (const char *format,
+                              gint64      time_ms);
+
 G_END_DECLS
 
 #endif /* __SHELL_UTIL_H__ */