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/loadSender.ts

@@ -0,0 +1,52 @@
+"use strict";
+
+import { stringify } from "./utils";
+
+import { ReceiverSelection
+       , ReceiverSelectorMediaType } from "../receiver_selectors";
+
+
+interface LoadSenderOptions {
+    tabId: number;
+    frameId: number;
+    selection: ReceiverSelection;
+}
+
+/**
+ * Loads the appropriate sender for a given receiver
+ * selector response.
+ */
+export default async function loadSender (opts: LoadSenderOptions) {
+    // Cancelled
+    if (!opts.selection) {
+        return;
+    }
+
+    switch (opts.selection.mediaType) {
+        case ReceiverSelectorMediaType.Tab:
+        case ReceiverSelectorMediaType.Screen: {
+            await browser.tabs.executeScript(opts.tabId, {
+                code: stringify`
+                    window.selectedMedia = ${opts.selection.mediaType};
+                    window.selectedReceiver = ${opts.selection.receiver};
+                `
+              , frameId: opts.frameId
+            });
+
+            await browser.tabs.executeScript(opts.tabId, {
+                file: "senders/mirroringCast.js"
+              , frameId: opts.frameId
+            });
+
+            break;
+        }
+
+        case ReceiverSelectorMediaType.File: {
+            const fileUrl = new URL(`file://${opts.selection.filePath}`);
+            const mediaSession = await mediaCasting.loadMediaUrl(
+                    fileUrl.href, opts.selection.receiver);
+
+            break;
+        }
+    }
+}