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/ui/popup/index.tsx

@@ -179,11 +179,6 @@ class PopupApp extends Component<{}, PopupAppState> {
             try {
                 const filePath = window.prompt();
 
-                // Validate URL
-                const fileUrl = new URL(filePath.startsWith("file://")
-                    ? filePath
-                    : `file://${filePath}`);
-
                 this.setState({
                     mediaType
                   , filePath