Allow mapping of floats in QGraphicsScene::mapToScene

The function QGraphicsScene::mapToScene is used to map screen coordinates to the coordinates in the scene, so it seems fine, that it only allows integer coordinates, because the mouse and windows are using integers.

BUT if your are using TabletEvents, you get subpixel precision (floats) for your input and at the moment it is not possible to transform such a position to scene coordinates.