Introduce "innerwiki" plugin

From the readme:

This plugin enables TiddlyWiki to embed a modified copy of itself (an "innerwiki"). The primary motivation is to be able to produce screenshot illustrations that are automatically up-to-date with the appearance of TiddlyWiki as it changes over time, or to produce the same screenshot in different languages

plugins/tiddlywiki/innerwiki/data.js

@@ -0,0 +1,56 @@
+/*\
+title: $:/plugins/tiddlywiki/innerwiki/data.js
+type: application/javascript
+module-type: widget
+
+Widget to represent a single item of data
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var Widget = require("$:/core/modules/widgets/widget.js").widget;
+
+var DataWidget = function(parseTreeNode,options) {
+	this.initialise(parseTreeNode,options);
+};
+
+/*
+Inherit from the base widget class
+*/
+DataWidget.prototype = new Widget();
+
+/*
+Render this widget into the DOM
+*/
+DataWidget.prototype.render = function(parent,nextSibling) {
+	this.parentDomNode = parent;
+	this.computeAttributes();
+	this.execute();
+	this.renderChildren(parent,nextSibling);
+};
+
+/*
+Compute the internal state of the widget
+*/
+DataWidget.prototype.execute = function() {
+	// Construct the child widgets
+	this.makeChildWidgets();
+};
+
+/*
+Selectively refreshes the widget if needed. Returns true if the widget or any of its children needed re-rendering
+*/
+DataWidget.prototype.refresh = function(changedTiddlers) {
+	// Refresh our attributes
+	var changedAttributes = this.computeAttributes();
+	// Refresh our children
+	return this.refreshChildren(changedTiddlers);
+};
+
+exports.data = DataWidget;
+
+})();