Guild - Qt interoperation

I've split the Actor class into "scheduler" and "actor" functionality. The latter is implemented as a mixin that can be used with other scheduling / event systems such as provided by Qt. The qtactor.py file contains a QtActorMixin that can be used with any PyQt class, allowing that class to have actor functionality and be used in guild pipelines.

There is also a simple ActorSignal component that converts guild pipeline outputs to Qt signals. This allows guild and Qt components to be connected without changing either of them.