Fixed a limited zooming bug in KoCanvasControllerWidget

When you tried to zoom-in/out with current zoom level equal to the
maximum/minimum allowed zoom level the canvas jumped, because the
preferred center used to change, but the zoom level not.

Now the setting of the new preferred center is part of
KoZoomController::setZoom so it can calculate the right value
of the offset basing on the real change of the zoom level.