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/utils.ts

@@ -9,6 +9,28 @@ export function getNextEllipsis (ellipsis: string): string {
     /* tslint:enable:curly */
 }
 
+/**
+ * Template literal tag function, JSON-encodes substitutions.
+ */
+export function stringify (
+        templateStrings: TemplateStringsArray
+      , ...substitutions: any[]) {
+
+    let formattedString = "";
+
+    for (const templateString of templateStrings) {
+        if (!formattedString) {
+            formattedString += templateString;
+            continue;
+        }
+
+        formattedString += JSON.stringify(substitutions.shift());
+        formattedString += templateString;
+    }
+
+    return formattedString;
+}
+
 
 interface WindowCenteredProps {
     width: number;