Now all the threading in KisBrushOp is done using strokes framework only,
therefore we can throttle CPU in a correct way.

The patch also includes:

1) KisStrokeStrategy::balancingRatioOverride() can now override the
   global preferred queues balancing ratio. Basically, when the paintop
   supports async updates we set updates higher priority over brush jobs.

2) KisPainter::runnableStrokeJobsInterface() now must hold a pointer to the
   currently preferred jobs execution engines.

   TODO: implement a fake default execution engine, tight now all the
         unittests will crash as soon as they try to paint with a paintop

3) KisBrush::threadingAllowed() now tells if the brush is allowed to
   use internals multithreading with QtConcurrent. It is forbidden for
   the asynchronous paintops, because it saturates CPU cores with extra
   threads.