Use the new PixmapSlider class which is based on Qt's AbstractSlider
class for sliders. This should also fix some offset problems in
PositionSlider which appeared after the introduction of the new Button
class
PixmapSlider is a subclass of QAbstractSlider.
It is independent of Skin.cpp. Slider is now only a wrapper to update the
QPixmaps on a skin change.
removed a workaround from equalizerwidget, that was necessary for the previous
implementation.
2-3 one line fixes I don't remember
