Fix screensaver to work as user expected, use xfce4-screensaver now

As described in DebianBug#1034607 and DebianBug#1082003, light-locker is not configurable in xfce4 now. We perfer xfce4-screensaver, instead.

And it is better to release this fix to trixie point release 13.2 (13.1 is scheduled at this weekend, too short).