Set up a callback system for the browser link.

This should somewhat simulate asynchronous method calling.

add-on/background.js

@@ -9,12 +9,5 @@ Listen for messages from the app.
 */
 port.onMessage.addListener((response) => {
   console.log("Received: " + response);
+  port.postMessage("pong");
 });
-
-/*
-On a click on the browser action, send the app a message.
-*/
-browser.browserAction.onClicked.addListener(() => {
-  console.log("Sending:  ping");
-  port.postMessage("ping");
-});