Because the CcBackgroundPaintable is now reusable, it can be used inside
the preview. This allows removing lots of code, since the drawing is now
done by the paintable. Because the scale factor was previously not
handled properly, this change makes the preview much sharper on scale
factors > 1.
Fixes#704
This is a dedicate class to generate the preview. Not only
it renders the pixbuf of the wallpaper, but it also adds
lock screen or desktop frames on top of it. It also monitors
the time to keep the lock screen preview label always updated.