[Bug 908513] fix shortcuts translations issue [r=crdlc]

https://bugzilla.mozilla.org/show_bug.cgi?id=908513

* Make sure to always use the most valid translation OR default to the original one if there's nothing more suitable.
* When sending to the API to update icons, they sometimes come back in a different order, so they're now reordered according to the user's original order.
* Changed "language changed" event to "localized", since sometimes the methods ran before the new translations were ready - which caused the code to use the old translation.