The design calls for the curtain to appear in the gdm greeter too.
Implement this by having the screenshield manage the login dialog
(delegating its creation to SessionMode).
https://bugzilla.gnome.org/show_bug.cgi?id=619955
This separates the screen shield into two main screens. One is
the lock screen, and it is shown when coming back from idle status
and when failing authentication. The other is the actual unlock
dialog.
Moving from the first to the second is possible by pressing Escape
or by dragging an arrow on the bottom on the screen.
https://bugzilla.gnome.org/show_bug.cgi?id=619955
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.
https://bugzilla.gnome.org/show_bug.cgi?id=619955
