Add XfceScreensaver
To share screensaver code between xfce4-power-manager and xfce4-session.
Except for the formatting, the screensaver code reproduced here is that
of xfce4-power-manager@8b2578c0. The code was the same between the two
components, except for the prioritization of org.xfce.ScreenSaver in
xfce_screensaver_init() and the fix in xfce4-power-manager@8b2578c0.
Related: xfce4-session#149 (comment 64581)
Edited by Gaël Bonithon