Add client code for handling submessages.

This commit lays the foundation to handle submessages for
plugin widgets.  Right now it just logs events, but subsequent
commits will add widget functionality.

static/js/echo.js

@@ -204,6 +204,8 @@ exports.process_from_server = function process_from_server(messages) {
         // the backend.
         client_message.timestamp = message.timestamp;
 
+        client_message.submessages = message.submessages;
+
         msgs_to_rerender.push(client_message);
         delete waiting_for_ack[client_message.id];
     });