15549487: Remove _initialize methods from Widget2

It is a good practice to prefer composition over inheritance. And
since we don't make widgets inheriting from other widgets anymore, we
don't have to separate the initialization of instance variables and
the initialization of subwidgets.

This is removing _initialize and _initializeSubwidgets in favor of
always using the object constructor instead.

src/Widget2.js

@@ -12,17 +12,17 @@ import { newId } from "./idGenerator.js";
  * @example
  *
  * class TitleWidget extends Widget {
- *   _initialize(spec) {
- *       this._title = spec.title || "Hello World";
+ *   constructor({ title }) {
+ *       this._title = title || "Hello World";
  *   }
  *
  *   renderContentOn(html) {
- *     html.h1(title);
+ *     html.h1(this._title);
  *   }
  *
  * };
  *
- * let helloWorldWidget = new TitleWidget({title: "Hello Widget!"});
+ * let helloWorldWidget = new TitleWidget({ title: "Hello Widget!" });
  *
  * $(document).ready(() => {
  *   helloWorldWidget.appendTo("BODY");
@@ -79,24 +79,8 @@ export default class Widget2 {
 		this._getParameters = this._router.getParameters;
 		this._getParameter = this._router.getParameter;
 		this._setParameters = this._router.setParameters;
-
-		this._initialize(...arguments);
-		this._initializeSubwidgets(...arguments);
 	}
 
-	/**
-	 * Hook evaluated at the beginning of initialization. Always
-	 * implement initialization code in overrides of this method (or
-	 * of `_initializeSubwidgets()`). In particular, don't override
-	 * `constructor()`.
-	 */
-	_initialize(_spec) {}
-
-	/**
-	 * Hook evaluated at the end of initialization.
-	 */
-	_initializeSubwidgets(_spec) {}
-
 	//
 	// Public
 	//