Drew suggested exposing a base class to subclass for providers and muxers, this way we may check inputs using instanceof.

Creates base classes for providers and muxers. All the classes are exposed by UrlbarUtils, for 2 reasons: 1. UrlbarProvidersManager on instantiation is importing provider modules, that may reimport the manager module. Exposing classes from it could cause a recursive failure. 2. It's nicer if only one of the urlbar modules exports multiple symbols (thus moving the QueryContext class here too)

Pushed by mak77@bonardo.net: https://hg.mozilla.org/integration/autoland/rev/c0f4a3b6b16d Expose base classes for providers and muxers. r=Standard8