ScreenShield: separate notifications for active and locked

Now that we have an explicit active-but-not-locked state, we should
use different signals to notify changes. lock-status-changed is
renamed to active-changed, and a new locked-changed is introduced.

https://bugzilla.gnome.org/show_bug.cgi?id=693007

js/ui/shellDBus.js

@@ -247,7 +247,7 @@ const ScreenSaverDBus = new Lang.Class({
         this.parent();
 
         this._screenShield = screenShield;
-        screenShield.connect('lock-status-changed', Lang.bind(this, function(shield) {
+        screenShield.connect('active-changed', Lang.bind(this, function(shield) {
             this._dbusImpl.emit_signal('ActiveChanged', GLib.Variant.new('(b)', [shield.active]));
         }));