power: Add a watchdog to keep X's builtin screen saver disabled

X's builtin screen saver may activate DPMS. Since we want to activate
DPMS ourselves according to our own policy we must make sure that X's
screen saver remains disabled.

This code is a copy of the original found in gs-watcher-x11.c from
gnome-screensaver which stopped being used in GNOME 3.6.

https://bugzilla.gnome.org/show_bug.cgi?id=686339