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.cpp

@@ -121,6 +121,3 @@ ToggleButton::setPixmaps(Skin *skin)
 	update();
 }
 
-ToggleButton::~ToggleButton ()
-{
-}