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-js.c

@@ -79,3 +79,15 @@ shell_js_add_extension_importer (const char  *target_object_script,
   JS_EndRequest (context);
   return ret;
 }
+
+/**
+ * shell_js_format_int_alternative_output:
+ * @intval:
+ *
+ * Returns: (transfer full):
+ */
+gchar *
+shell_js_format_int_alternative_output (gint intval)
+{
+  return g_strdup_printf ("%Id", intval);
+}