Allow multiple addon repository providers to be added

Presently, when searching for addons and such, Songbird only shows addons available from the official addons.songbirdnest.com page only.

Pioneers of the Inevitable should provide a way for manufacturers such as Sansa to start their own repository which users can easily supplement the official one with, and properly integrate it into Songbird. And third party developers should be able to host their own too (certain codec's aren't covered by patents in some countries for instance, so developers in such countries may like a good way to offer those addons, where they are legal).

Another huge advantage of this, is that Pioneers of The Inevitable could keep their official plugins (ones designed by the Songbird team), in a separate addons page to community ones.