Bug 837494 - re-implement `contentStyle*` using the new nsIDOMWindowUtils methods

 - added `mod` module, with `attach`, `detach` and `getWindowTarget`
 - added `loadSheet` and `removeSheet` to `window/utils`
 - added `values` method to `util/object`
 - replace usage of stylesheet services with `attach`/`detach` a `Style` object