Support for blur behind plasmoids

Add a way to access the wallpaper for the plasmoid that can be used to put effects on it (plamo will also use that for having a support to horizontally scrolling wallpaper) And add new elements in the theme to explicitly support blur. widgets/background has 2 new prefixes: brurred and blurred-mask used to render semitransparent widgets with blurred background, while the normal background is fully opaque.