make sawfish.wm.ext.wallpaper more robust against non-existing root-wallpaper or wallpaper-setter
more robustness for image widget aswell (only update the preview if the image actually exists),
also ensure set-wallpaper is only called, when init-wallpaper is set (might get called from sawfish-config :after-set even if init-wallpaper is unset),
check wallpaper-setter properly against program-exists-p or file-exists-p (the former check might be useful, when the value is set from ~.sawfish/rc with only the application name, without full path - the latter is the valid check when wallpaper-setter is set from sawfish-config)