Moved most of the message handling to other classes.

I'm finding that this code is being used multiple times, so inheritance seems to be warranted.

add-on/background.js

@@ -4,10 +4,42 @@ On startup, connect to the "ping_pong" app.
 */
 var port = browser.runtime.connectNative("universalmusic");
 
+var listeners = [function (message, returnValue) {
+    if (returnValue == null)
+    {
+        returnValue = "pong";
+    }
+    return returnValue;
+}];
+
+function sleep(ms) {
+    return new Promise(resolve => setTimeout(resolve, ms));
+}
+
+function handleMessage(message)
+{
+    return new Promise((resolve, reject) => {
+        var returnValue = null;
+        for (index in listeners)
+        {
+            returnValue = listeners[index](message, returnValue);
+        }
+        resolve(returnValue);
+    })
+}
+
 /*
-Listen for messages from the app.
-*/
-port.onMessage.addListener((response) => {
-  console.log("Received: " + response);
-  port.postMessage("pong");
+ * Listen for messages from the app.
+ */
+port.onMessage.addListener((message) => {
+    console.log("Received: ", message);
+
+    handleMessage(message.message).then((response) => {
+        returnValue = {
+            "messageNum": message.messageNum,
+            "message": response
+        }
+        console.log("Sending ", returnValue)
+        port.postMessage(returnValue);
+    });
 });