Declarations and implementations of MixerWorkerThread and ThreadableJob
have been moved into separate source files.
Furthermore there were some improvements to MixerWorkerThreads.
MixerWorkerThread::processJobQueue() does not return until the job
queue completely has been processed. This way each thread can "help"
to finish processing the queue and does not get back to sleep until
all of the work is done.
Management of the queue is now done via an array of QAtomicPointers.
Items that are non-NULL still need to be processed while NULL-items
were taken from the queue (i.e. in progress or done). Thus we do not
need to deal with ThreadableJob-states within MixerWorkerThread anymore.
