Adds the ability to play youtube videos.

It still needs a bit of work, but playing and pausing are operational.

add-on/foreground.js

@@ -0,0 +1,82 @@
+console.debug("Loading foreground.js")
+let background;
+
+function handleMessage(message)
+{
+    if (typeof message == "object" && "type" in message)
+    {
+        let type = message.type;
+        lastPeriod = type.lastIndexOf(".");
+        if (lastPeriod != -1)
+        {
+            type = type.substring(lastPeriod + 1);
+        }
+        switch (type)
+        {
+        case "QueryStatus":
+            return getState();
+        case "QueryTime":
+            return getTime();
+        case "QueryLength":
+            return getLength();
+        case "CommandSetPlayback":
+            switch (message.status)
+            {
+            case "PLAY":
+                return play();
+            case "PAUSE":
+                return pause();
+            }
+            return false;
+        case "CommandSeek":
+            return seek(message.time);
+        }
+    }
+}
+
+function sendUpdate(response)
+{
+    post = data => background.postMessage({
+        type: "update",
+        data: data
+    });
+    if (response instanceof Promise)
+    {
+        response.then(post);
+    }
+    else
+    {
+        post(response);
+    }
+}
+
+background = browser.runtime.connect({name:"universalMusic"});
+background.onMessage.addListener(message => {
+    let num = message.num;
+    response = handleMessage(message.data);
+    post = data => background.postMessage({
+        type: "response",
+        num: num,
+        data: data
+    });
+    if (response instanceof Promise)
+    {
+        response.then(post);
+    }
+    else
+    {
+        post(response);
+    }
+});
+
+if (document.readyState === "complete")
+{
+    sendUpdate("loaded");
+}
+else
+{
+    sendUpdate("loaded");
+    window.addEventListener("load", () => {
+        sendUpdate("loaded");
+    });
+}