Fix one crash on exit, caused by double-freeing QWidgets stored in a private d-pointer class of another QWidget.

Dear Amarok devs, please don't _ever_ use d-pointers in QWidgets. Here is an article that explains the dangers of doing so:

<a href="http://www.purinchu.net/wp/2009/02/04/another-programming-tidbit/">http://www.purinchu.net/wp/2009/02/04/another-programming-tidbit/</a>;