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.
It will become essentially what CcBackgroundChooserDialog,
but without a dialog and organized differently. Right now,
it is as minimal as it could be and only shows wallpapers.
As we grow it, it may cover pictures and even colors.
