Decoration is in fact quite a simple process that is applied on entry
to rendering: we take a base renderer, transform it with a decorator,
and then render a report with it. The new render.Decorate() function
does exactly that.

There is one exception. When rendering an individual node, e.g. for
showing its details panel in the UI, we must not lose the node during
decoration. That requires some special logic, which previously resided
in the PreciousNodeRenderer, and now lives in handleNode.