Add UnlockDialog for unlocking the screen shield

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

js/ui/sessionMode.js

@@ -39,7 +39,7 @@ const _modes = {
              hasRunDialog: false,
              hasWorkspaces: false,
              createSession: Main.createGDMSession,
-             extraStylesheet: global.datadir + '/theme/gdm.css',
+             extraStylesheet: null,
              statusArea: {
                  order: [
                      'a11y', 'display', 'keyboard',