Ensure that the lightbox is above everything (including the screenlock
itself) when fading in - this allows for fading while showing the
unlock dialog. Also, don't pushModal again when already locked, or
we won't get out of it.
When the screenshield is deactivated, instead of going back to the
session immediately, prompt the user for authentication.
This essentially reinstates what used to be provided by gnome-screensaver.
We are replacing the gnome-screensaver module with with a screen shield
that is part of gnome-shell.
This patch fades out the screen on idle and shows a shield with a background
image when there is activity again. The shield can be removed with a key or
button press.
