Fix CodeMirror on fake dom (#3547)

* fix for #3547

see https://developer.mozilla.org/de/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty

because of "strict mode" - the `parentNode` property may not be writable and causes an error in `fakedom.js` (appendChild)

to reproduce: install CodeMirror, install "Tools for exploring internals of TW", open a heavy Tiddler like the Control-Panel in edit-mode, toggle preview on, switch to "parse tree" or "widget tree"

* check for tw fakedom

plugins/tiddlywiki/codemirror/engine.js

@@ -109,7 +109,9 @@ function CodeMirrorEngine(options) {
 	// Create the CodeMirror instance
 	this.cm = window.CodeMirror(function(cmDomNode) {
 		// Note that this is a synchronous callback that is called before the constructor returns
-		self.domNode.appendChild(cmDomNode);
+		if(!self.widget.document.isTiddlyWikiFakeDom) {
+			self.domNode.appendChild(cmDomNode);
+		}
 	},config);
 
 	// Set up a change event handler