Add systemd sleep inhibitor (bug #15929)

Looking at the timestamps in debug output it seems to me that systemd is simply suspending too fast for xfce4-screensaver to activate the lock screen.

Adding a sleep delay inhibitor lock makes the problem go away since it gives xfce4-screensaver time to activate the lock screen before allowing systemd to continue suspending the system.

Signed-off-by: Sean Davis <smd.seandavis@gmail.com>