screenShield: Do not wake up screen after adding hidden source

The screen should be woken up when a new notification is shown on
the lock screen, but not when a notification arrives while disabled.
Add a missing condition to fix.

https://bugzilla.gnome.org/show_bug.cgi?id=744114

js/ui/screenShield.js

@@ -271,7 +271,8 @@ const NotificationsBox = new Lang.Class({
                              });
 
             this._updateVisibility();
-            this.emit('wake-up-screen');
+            if (obj.sourceBox.visible)
+                this.emit('wake-up-screen');
         }
     },