format: move shell_format_int_alternative_output() to ShellJS

gnome-shell-extension-prefs uses format(), but can't pull in Shell
(which is a dependency for the module), since that in turn would pull in
Meta. Fix this by moving the introspected int format function to ShellJS
instead.

https://bugzilla.gnome.org/show_bug.cgi?id=673106

src/shell-util.h

@@ -52,8 +52,6 @@ gboolean shell_session_is_active_for_systemd (void);
 gboolean shell_util_wifexited                  (int               status,
                                                 int              *exit);
 
-gchar    *shell_format_int_alternative_output  (gint              intval);
-
 
 G_END_DECLS