Also I think I fixed a memory leak if the cover could not be loaded from its file for some reason.
Remember, pointers bad, RAII (http://www.hackcraft.net/raii/) good!

Large covers are no longer cached since the QPixmapCache is application-global.  But this is OK,
large covers are hardly ever shown.

svn path=/trunk/KDE/kdemultimedia/juk/; revision=832077