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/SelectorManager.ts

@@ -0,0 +1,85 @@
+"use strict";
+
+import options from "./lib/options";
+
+import { getReceiverSelector
+       , ReceiverSelection
+       , ReceiverSelector
+       , ReceiverSelectorMediaType
+       , ReceiverSelectorType } from "./receiver_selectors";
+
+import StatusManager from "./StatusManager";
+
+
+let sharedSelector: ReceiverSelector;
+
+async function getSelector () {
+    return getReceiverSelector(
+            await options.get("receiverSelectorType"));
+}
+
+async function getSharedSelector () {
+    if (!sharedSelector) {
+        sharedSelector = await getSelector();
+    }
+
+    return sharedSelector;
+}
+
+/**
+ * Opens a receiver selector with the specified
+ * default/available media types.
+ *
+ * Returns a promise that:
+ *   - Resolves to a ReceiverSelection object if selection is
+ *      successful.
+ *   - Resolves to null if the selection is cancelled.
+ *   - Rejects if the selection fails.
+ */
+async function getSelection (
+        defaultMediaType =
+                ReceiverSelectorMediaType.Tab
+      , availableMediaTypes =
+                ReceiverSelectorMediaType.Tab
+              | ReceiverSelectorMediaType.Screen)
+              // | ReceiverSelectorMediaType.File)
+        : Promise<ReceiverSelection> {
+
+    return new Promise(async (resolve, reject) => {
+        /**
+         * Close any existing selector, and renew to minimize issues
+         * with bridge failing.
+         */
+        await getSharedSelector();
+        if (sharedSelector.isOpen) {
+            sharedSelector.close();
+        }
+
+        sharedSelector = await getSelector();
+
+        sharedSelector.addEventListener("selected", ev => {
+            console.info("fx_cast (Debug): Selected receiver", ev.detail);
+            resolve(ev.detail);
+        });
+
+        sharedSelector.addEventListener("cancelled", ev => {
+            console.info("fx_cast (Debug): Cancelled receiver selection");
+            resolve(null);
+        });
+
+        sharedSelector.addEventListener("error", ev => {
+            console.error("fx_cast (Debug): Failed to select receiver");
+            reject();
+        });
+
+        sharedSelector.open(
+                StatusManager.getReceivers()
+              , defaultMediaType
+              , availableMediaTypes);
+    });
+}
+
+export default {
+    getSelection
+  , getSharedSelector
+};