xdg-screensaver: fix support for freedesktop DBus APIs, and use it for recent GNOME 3

Recent GNOME 3 does not have gnome-screensaver installed by default, and is no longer correctly detected by xdg-screensaver. Fix this, and also fix the freedesktop screensaver methods on which it must now depend.

I offer MR !62 (merged) to fix this issue.