Starting to make the members of $tw.Wiki be private

We want to avoid plugins from directly accessing the tiddlers hashmap.
Later we’ll do pluginTiddlers, pluginInfo and shadowTiddlers.
Freeze array or object tiddler fields

Previously object fields like the tags array weren’t frozen.
Make shadowTiddlers, pluginTiddlers and pluginInfo be private to the Wiki object constructor
New error message text for recursive transclusion error