Qt: correctly restore the playlistWidget state when the undocked playlist is closed

Close #3607
(cherry picked from commit 97a0b3ba067f45612975276e33c684b5b249a6c4)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/gui/qt4/components/playlist/playlist.cpp

@@ -156,6 +156,7 @@ void PlaylistWidget::closeEvent( QCloseEvent *event )
     }
     else
     {
+        p_intf->p_sys->p_mi->playlistVisible = false;
         hide();
         event->ignore();
     }