Decorators, begone!

[rade]

Decorators turn out to be an unnecessary abstraction that rather obscures what's going on. So let's get rid of them!

Decorators were used to dynamically construct filters from UI selections - e.g. namespaces, system vs app containers, etc; the selectors in the bottom left corner of the current scope UI - and apply them to the output of topology renderers - pods, containers-by-image, etc; the selectors at the top centre of the current scope UI. Decorators were accomplishing that in a rather complicated way, by composing renderers that wrap the base topology renderers.

This PR unravels all that.

The filter selection from the UI is now turned into a FilterFunc, which we Apply() to the output of topology renderers. The render.Render() function does exactly that, and very clearly so. The one place where we need to do something special is when rendering a single node, e.g. for the details panel in the UI. See comments in the handleNode code.

The PR looks larger than it actually is. There are a bunch of mechanical changes that touch several files, especially the removal of Decorators from the Renderer.Render() signature, and use of the new render.Render() function. The individual commits are self-contained, but the first one is very much an intermediate step which is largely superseded by later commits. Also note that GH does a poor job of showing the diff for the change that moves the filter logic to FilterFunc.Apply().

[rade]

I've tested the correctness of this by running

echo > report.json
for r in processes containers pods hosts \
         "processes?unconnected=hide" \
         "containers?pseudo=hide&namespace=default,scope&system=application&stopped=running" \
         "pods?pseudo=hide&namespace=default,scope" ; do \
    echo $r
    curl -s "http://localhost:4040/api/topology/$r" | jq -S '.' >> report.json; \
done

against master and this branch and diffing the resulting reports.

[bboreham]

Very nice.

I think I would have broken the steps down a little more, e.g. the refactoring to extract isNotBar in render/filters_test.go can stand alone.