Add unread count in window title

The window title now shows the global number of unread messages as
"Signal (1)". This way the user can see the number of unread messages
in the task bar and when alt-tabbing.

Resolves: #384

js/background.js

@@ -172,8 +172,6 @@
             type           : 'incoming'
         });
 
-        extension.navigator.setBadgeText(newUnreadCount);
-
         return message;
     }