The ScreenPreviewWidget doesn't make much sense any more for the general
useage as it's not QML based and thus not suited for any usage in Plasma.
This means that features like previewing the Plasma wallpaper cannot
work any more.
The Widget is only used in a few KCMs (KDM, Screensavers, Screenedges).
Currently we expect that KDM and legacy X Screensavers won't make it into
the next release, it makes sense to just move the widget to screenedges
where it's still useful.
This change has been discussed with notmart.
