This commit implements *true* alpha-blend fading for ContextBoxes in GraphicsItemFader, and it's fast too. This means that varying backgrounds like gradients and the like should correctly render. This does mean that it won't work for pixmaps, so if we want to fade pixmaps we'd have to figure something else out for them (although pixmaps contained in ContextBoxes should probably fade), or have the old method alongside.

One issue: Switching from QGraphicsTextItem to QGraphicsSimpleTextItem causes the title bars not to display...haven't figured out why yet.