This adds the ShadowedImage and ShadowedTexture items. The first is a
QML-only item that makes use of the second. The second is a
ShadowedRectangle that uses a texture as color source for the item
instead of just a color.
It also does some cleanup on the scenegraph/shader related bits
introduced with ShadowedRectangle. Most importantly, it removes the
mostly duplicated core shaders in favour of a simple solution using