Implement multitouch painting in krita

This moves the multihand painting code into the freehand tool helper
and reorganizes the data to take care of multiple input points per
event, or generated output points (for multihand). The api is now much
simpler, the code a bit more complex.

Limitations:

* additional touch points during painting are not detected
* dropping touch points during painting is not detected
* it's not tested because the canvas doesn't show up for me.