With only a few SVG elements in use, at an essentially fixed size, a cache
becomes highly overkill (and even if not, KPixmapCache has issues, although I
am biased...), so just use a simple QMap to cache generated SVG elements.

I could probably go farther (e.g. remove locking) and I'm sure there's headers
that could be removed, but I think this is fairly low-fat again in keeping with
the spirit of the code committed yesterday. :-)

svn path=/trunk/KDE/kdemultimedia/juk/; revision=1231346