rework PlaylistWindow: move handling of visibilitychanges into Playlistwindow

and rely on signals and slots to get rid of MainWindow::togglePL().
cleaned up some unnecessary includes

src/Button.h

@@ -43,7 +43,7 @@ class ToggleButton : public PixmapButton
 	Q_OBJECT
 	public:
 		ToggleButton (QWidget *parent, uint, uint, uint, uint);
-		~ToggleButton ();
+		~ToggleButton () {}
 
 	public slots:
 		void setPixmaps(Skin *skin);