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/lib/endpoints.ts

@@ -0,0 +1,45 @@
+"use strict";
+
+/**
+ * Cast sender API loader script URL.
+ *
+ * Since the actual cast sender API script is hosted locally
+ * within Chrome, this script just acts a loader script for
+ * the real script whilst also doing some UA string
+ * checking.
+ */
+export const CAST_LOADER_SCRIPT_URL =
+        "https://www.gstatic.com/cv/js/sender/v1/cast_sender.js";
+
+/**
+ * Framework API loader script URL.
+ *
+ * Same URL as the usual loader script, but the additional
+ * search parameter is checked from within the script and
+ * the framework API script is conditionally loaded in
+ * addition to the regular API script.
+ */
+export const CAST_FRAMEWORK_LOADER_SCRIPT_URL =
+        `${CAST_LOADER_SCRIPT_URL}?loadCastFramework=1`;
+
+/**
+ * Cast API script URLs.
+ *
+ * Cast functionality in Chrome was previously provided by
+ * an extension. The cast API script is still provided via
+ * chrome-extension URLs for compatibility reasons.
+ */
+export const CAST_SCRIPT_URLS = [
+    "chrome-extension://pkedcjkdefgpdelpbcmbmeomcjbeemfm/cast_sender.js"
+  , "chrome-extension://enhhojjnijigcajfphajepfemndkmdlo/cast_sender.js"
+];
+
+/**
+ * Framework API script URL.
+ *
+ * Unlike the basic cast sender API, the framework API is
+ * not hosted locally within Chrome and is the only script
+ * fetched directly from Google servers.
+ */
+export const CAST_FRAMEWORK_SCRIPT_URL =
+        "https://www.gstatic.com/cast/sdk/libs/sender/1.0/cast_framework.js";