Wrap KoShapeBackground in QPointer

This patch wraps KoShapeBackground in a QPointer instead of doing manual
refcounting. Bug 321714 shows, imo, that the manual refcounting is
fragile and sometimes breaks. I don't remember why we weren't supposed
to use QPointer (other than that it gets a little ugly with subclassing),
but it seems a good fit here.

There are other classes that do manual refcounting, and I think those
should be converted, too -- but in another patch.