ScreenShield: when explicitly locking, tween the shield from the top

The curtain should appear to be an overlay on top of the system,
and since it is lifted by dragging up, it makes sense to slide it
down on lock.

https://bugzilla.gnome.org/show_bug.cgi?id=681143

data/theme/gnome-shell.css

@@ -2201,12 +2201,12 @@ StButton.popup-menu-item:insensitive {
 
 /* Screen shield */
 
-#screenShieldGroup {
+#lockDialogGroup {
     background: #2e3436 url(noise-texture.png);
     background-repeat: repeat;
 }
 
-#screenShieldGroup .arrow {
+#lockScreenGroup .arrow {
     color: #333333;
     width: 100px;
     height: 50px;