What we have at the moment isn't really the same as TiddlyWiki
classic's shadow tiddlers, it's a much simpler system for excluding
tiddlers. We'll use the term "shadow" instead to refer to the way that
tiddlers in plugins behave, which is exactly like TiddlyWiki classic's
shadow tiddlers.
Thus introducing a few functional regressions, but it's going to be
easier to fix things up without the old code knocking around and
getting in the way.
