major refactoring of things:

* consistent naming of member variables
* rely on QGraphicsItem to do the caching for us and make paintInterface a naive painter
* make the SlideShow tell the outside world when there is a new image to show
* let the SlideShow handle potd dataengine updates
* don't hold onto the source image in memory for longer than needed

so now the SlideShow handles all the image loading and the Applet just paints whenever it's told to.

the performance increase is impressive and instantly recognizable: try rotating or dragging an image frame around the desktop.