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
For those who like their system pure, this provides the ability to purge a
pesky extension and its precious place on your disk space, and in your
"Local Extension" list.
https://bugzilla.gnome.org/show_bug.cgi?id=658612
Conflicts:
js/ui/extensionSystem.js
