Recalcualtes the slider scene for this widget.

def recalculate(self):
        """
        Recalcualtes the slider scene for this widget.
        """
        # recalculate the scene geometry
        scene = self.scene()
        w = self.calculateSceneWidth()
        scene.setSceneRect(0, 0, w, self.height())
        
        # recalculate the item layout
        spacing = self.spacing()
        x = self.width() / 4.0
        y = self.height() / 2.0
        for item in self.items():
            pmap = item.pixmap()
            item.setPos(x, y - pmap.height() / 1.5)
            x += pmap.size().width() + spacing