*.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/misc/config.js.in

@@ -1,4 +1,5 @@
-/* mode: js2; indent-tabs-mode: nil; tab-size: 4 */
+// -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
+
 /* The name of this package (not localized) */
 const PACKAGE_NAME = '@PACKAGE_NAME@';
 /* The version of this package */