Bug 1049371 - verbose permissions for developers (first prototype)

This is a first prototype, to show how I'd solve this.
I'm especially unsure about how to notify the panel of a settings change and whether to refresh it. See [this hunk](https://github.com/mozfreddyb/gaia/compare/dev-permissions?expand=1#diff-e1e59a664daa46d3ffd67195db21934aR43)
Feedback appreciated.
Then we could simply use `this._isValidPerm` here.