*.js: Make emacs modelines consistent

js2-mode is no longer developed and we recommend js-mode these days,
so switch the modelines to specify that, and make them consistent
across all files.

https://bugzilla.gnome.org/show_bug.cgi?id=660358

js/ui/modalDialog.js

@@ -1,4 +1,4 @@
-/* -*- mode: js2; js2-basic-offset: 4; indent-tabs-mode: nil -*- */
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
 const Clutter = imports.gi.Clutter;
 const Gdk = imports.gi.Gdk;