Widgets/Styles: simplify usage of QPixmapCache

The styles, especially fusion style, is using QPixmapCache various ways. There is even a macro to simplify the usage.
All of them lack a generic usage without all the boilerplate code needed around.
The idea is to add a new private class QCachedPainter which is responsible for the caching through QPixmapCache and transparently redirects the painting code to the pixmap for QPixmapCache.