App loading screen: show messages processed so far

Also, show the same loading screen on index.js before we've bootstrapped
the app.

FREEBIE

js/background.js

@@ -123,6 +123,7 @@
         messageReceiver.addEventListener('verified', onVerified);
         messageReceiver.addEventListener('error', onError);
         messageReceiver.addEventListener('empty', onEmpty);
+        messageReceiver.addEventListener('progress', onProgress);
 
         window.textsecure.messaging = new textsecure.MessageSender(
             SERVER_URL, SERVER_PORTS, USERNAME, PASSWORD
@@ -158,6 +159,14 @@
             }
         }, 500);
     }
+    function onProgress(ev) {
+        var count = ev.count;
+
+        var view = window.owsDesktopApp.inboxView;
+        if (view) {
+            view.onProgress(count);
+        }
+    }
 
     function onContactReceived(ev) {
         var details = ev.contactDetails;