Replace XTestFakeKeyEvent with inhibit/heartbeat (Bug #11083)

Instead of using XTestFakeKeyEvent, attempt to contact whatever
screensaver is running via its dbus interface. Failing that,
a /xfce4-power-manager/heartbeat-command xfconf property will be
executed every 30 seconds to keep the screensaver from
launching. This way xscreensaver, xdg-screensaver, caffeine or
a similar tool can be used.