Restructure background script (#70)

Splits some background script functionality into separate modules:
 - Receiver selector handling is moved to ./SelectorManager.
 - Status bridge handling is moved to ./StatusManager.
 - Menu creation and updates are handled in ./createMenus.
 - Shim creation is handled in ./createShim.

TypedEventTarget allows EventTarget-derived classes to export typed events.

Options type definition is moved to ./lib/options, module assumes more responsibility for update handling and provides a "changed" event.

Private cast._requestSession method allows bypassing receiver selector.

ext/src/senders/mirroringCast.ts

@@ -1,15 +1,19 @@
 "use strict";
 
 import options from "../lib/options";
-import cast, { init } from "../shim/export";
+import cast, { ensureInit } from "../shim/export";
 
 import { ReceiverSelectorMediaType }
         from "../receiver_selectors/ReceiverSelector";
 
+import { Receiver } from "../types";
+
 
 // Variables passed from background
-const { selectedMedia }
-    : { selectedMedia: ReceiverSelectorMediaType } = (window as any);
+const { selectedMedia
+      , selectedReceiver }
+    : { selectedMedia: ReceiverSelectorMediaType
+      , selectedReceiver: Receiver } = (window as any);
 
 
 const FX_CAST_RECEIVER_APP_NAMESPACE = "urn:x-cast:fx_cast";
@@ -42,6 +46,10 @@ if (typeof navigator.mediaDevices.getDisplayMedia === "undefined") {
  * receiver device.
  */
 function sendAppMessage (subject: string, data: any) {
+    if (!session) {
+        return;
+    }
+
     session.sendMessage(FX_CAST_RECEIVER_APP_NAMESPACE, {
         subject
       , data
@@ -54,9 +62,7 @@ window.addEventListener("beforeunload", () => {
 });
 
 
-async function onRequestSessionSuccess (
-        newSession: cast.Session
-      , newSelectedMedia: ReceiverSelectorMediaType) {
+async function onRequestSessionSuccess (newSession: cast.Session) {
 
     cast.logMessage("onRequestSessionSuccess");
 
@@ -83,7 +89,7 @@ async function onRequestSessionSuccess (
         sendAppMessage("iceCandidate", ev.candidate);
     });
 
-    switch (newSelectedMedia) {
+    switch (selectedMedia) {
         case ReceiverSelectorMediaType.Tab: {
             const canvas = document.createElement("canvas");
             const ctx = canvas.getContext("2d");
@@ -161,8 +167,9 @@ function receiverListener (availability: string) {
 
     if (availability === cast.ReceiverAvailability.AVAILABLE) {
         wasSessionRequested = true;
-        cast.requestSession(
-                onRequestSessionSuccess
+        cast._requestSession(
+                selectedReceiver
+              , onRequestSessionSuccess
               , onRequestSessionError);
     }
 }
@@ -182,13 +189,7 @@ function onInitializeError () {
 }
 
 
-init().then(async bridgeInfo => {
-    if (!bridgeInfo.isVersionCompatible) {
-        console.error("__onGCastApiAvailable error");
-        return;
-    }
-
-
+ensureInit().then(async () => {
     const mirroringAppId = await options.get("mirroringAppId");
     const sessionRequest = new cast.SessionRequest(mirroringAppId);
 
@@ -196,9 +197,7 @@ init().then(async bridgeInfo => {
             sessionRequest
           , sessionListener
           , receiverListener
-          , undefined, undefined
-          , selectedMedia
-          , availableMediaTypes);
+          , undefined, undefined);
 
     cast.initialize(apiConfig
           , onInitializeSuccess