In Mixer, the old C-macro based code has been replaced by an OOP-like
design. Management of job queue now happens via some static member
methods of MixerWorkerThread. All the moved code still needs to be
splitted into some new files but here's a first dirty version.
All objects that are intended to be processed by MixerWorkerThreads
have to inherit ThreadableJob (name of class is subject of change).
One can add jobs to the job queue even if the queue is already being
processed. This is merely important for multithreading with upcoming
FX sends support.
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