Implement the first version of the multithreaded Pixel Brush

All the presets that use Pixel Brush (KisBrushOp) are now
multithreaded and rendered asynchronously. Basically, it means
that if the brush is too slow, Krita will lower down FPS rate
for the sake of faster rendering of the stroke.


Short summary:

1) It doesn't use strokes system's threading, just QtConcurrent. It
   is not good, but works for now. I hope it is only a temporary
   solution.

2) Updates are coming asynchronously with the period of 20...80ms,
   which is 50...12fps. I didn't manage to implement a correct control
   loop for auto-adjusting the FPS value, because it needs porting the
   threading part into strokes system and a bit of refactoring of
   the strokes system itself. Therefore, the FPS adjustment is controlled
   by an open-loop system, based on one-dab-rendering-time. Basically,
   FPS is proportional to the time spent on rendering a single tile.

3) The patch adds two new API functions: KisPaintOpSettings::
   needsAsynchronousUpdates() tells if the paintop uses threading and
   needs asynchronous updates. When this function returns true, the
   freehand stroke does additional calls to
   KisPaintOp::doAsyncronousUpdate(), which does the rendering itself.

4) Still to be implemented:
     * color source options
     * postprocessing: sharpness and texturing
     * selection handling (works only in Wash mode)
     * mirroring mode
     * pipe brushes

CC:kimageshop@kde.org