Added a PixmapSlider class and changed Slider to inherit from it.

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

src/equalizer/equalizerwidget.h

@@ -38,10 +38,10 @@ class EqualizerSlider : public Slider
 	EqualizerSlider (QWidget*, uint, uint, uint, uint, int, int, int);
 
 	signals:
-	void numberedValueChanged (int value, int id);
+	void numberedSliderMoved (int value, int id);
 
 	protected slots:
-	void on_self_value_changed (int value);
+	void on_self_slider_moved (int value);
 
 	private:
 	int m_id;