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/bridge.ts

@@ -0,0 +1,90 @@
+"use strict";
+
+import semver from "semver";
+
+import nativeMessaging from "./nativeMessaging";
+import options from "./options";
+
+
+async function connect (): Promise<browser.runtime.Port> {
+    const applicationName = await options.get("bridgeApplicationName");
+    const bridgePort = nativeMessaging.connectNative(applicationName);
+
+    bridgePort.onDisconnect.addListener(() => {
+        if (bridgePort.error) {
+            console.error(`${applicationName} disconnected:`
+                  , this.bridgePort.error.message);
+        } else {
+            console.info(`${applicationName} disconnected`);
+        }
+    });
+
+    return bridgePort;
+}
+
+
+export interface BridgeInfo {
+    name: string;
+    version: string;
+    expectedVersion: string;
+    isVersionExact: boolean;
+    isVersionCompatible: boolean;
+    isVersionOlder: boolean;
+    isVersionNewer: boolean;
+}
+
+async function getInfo (): Promise<BridgeInfo> {
+    const applicationName = await options.get("bridgeApplicationName");
+    let applicationVersion: string;
+
+    try {
+        const { version } = browser.runtime.getManifest();
+
+        applicationVersion = await nativeMessaging.sendNativeMessage(
+                applicationName
+              , { subject: "bridge:/getInfo"
+                , data: version });
+    } catch (err) {
+        return null;
+    }
+
+    /**
+     * If the target version is above 0.x.x range, API is stable
+     * and versions with minor or patch level changes should be
+     * compatible.
+     */
+    const isVersionCompatible =
+            semver.eq(applicationVersion, APPLICATION_VERSION)
+         || semver.diff(applicationVersion, APPLICATION_VERSION) !== "major"
+         && semver.major(APPLICATION_VERSION) !== 0;
+
+    const isVersionExact = semver.eq(applicationVersion, APPLICATION_VERSION);
+    const isVersionOlder = semver.lt(applicationVersion, APPLICATION_VERSION);
+    const isVersionNewer = semver.gt(applicationVersion, APPLICATION_VERSION);
+
+    // Print compatibility info to console
+    if (!isVersionCompatible) {
+        console.error(`Expecting ${applicationName} v${APPLICATION_VERSION}, found v${applicationVersion}.`
+              , isVersionOlder
+                    ? "Try updating the native app to the latest version."
+                    : "Try updating the extension to the latest version");
+    }
+
+    return {
+        name: applicationName
+      , version: applicationVersion
+      , expectedVersion: APPLICATION_VERSION
+
+        // Version info
+      , isVersionExact
+      , isVersionCompatible
+      , isVersionOlder
+      , isVersionNewer
+    };
+}
+
+
+export default {
+    connect
+  , getInfo
+};