Work around x errors by asking dialog to die on cancel

Basically, what is happening is that gnome-screensaver-dialog exits after the
5th failed attempt at unlocking the screen, but before the shake animation
finishes. If the timing is slightly unlucky, this results in gnome-screensaver
accessing X resources that have already been destroyed (I ran it through
xtrace, and that showed this happening)

My patch fixes this by making gnome-screensaver-dialog request to
gnome-screensaver that it be terminated after the 5th failed attempt (rather
than exitting straight away, although there is a fallback timeout too).
gnome-screensaver then terminates the dialog after it is finished with the
shake animation, to avoid the race condition that is currently making it crash.