screenShield: focus login screen after lifting shield

This commit ensures the login screen gets focused after
the screen shield is raised.

The code affects the unlock screen as well, but it's
less important since the unlock screen gets destroyed
and recreated each time the curtain moves, so it
has an opportunity to take focus on its own.

https://bugzilla.gnome.org/show_bug.cgi?id=708105

js/ui/screenShield.js

@@ -919,6 +919,9 @@ const ScreenShield = new Lang.Class({
 
         this._lockScreenState = MessageTray.State.HIDDEN;
         this._lockScreenGroup.hide();
+
+        this._dialog.actor.grab_key_focus();
+        this._dialog.actor.navigate_focus(null, Gtk.DirectionType.TAB_FORWARD, false);
     },
 
     _hideLockScreen: function(animate, velocity) {