Removes the functionality for Plasma::Wallpapers as that doesn't exist
any more and is not needed by the Screenedges KCM. The KCM still has
way too many functionality, but that can stay for the moment as it's
probably best to reimplement the monitor and the actions in QML.
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.
