Add mixin support

[marcoscaceres]

Seems we don't support mixins yet, right?

interface mixin Observable {
  void addEventListener(DOMString type,
                        EventListener listener,
                        boolean useCapture);
  // ...
};

[marcoscaceres]

@saschanaz, you feel like tackling this?

[saschanaz]

Yes 😄

[marcoscaceres]

Woot woot! 🎉 Anything I can do to help, just ping!