The `shouldShow` method always marked a step as seen, and having the
side effect there is a little confusing. Because we call shouldShow in
the StepPre, StepPost and Output handlers, its also hard to ensure an
invarant I think we want, which is that in the Post and Output
handlers, we've already seen the event.
So, let's move the marking out of `shouldShow` and into
`OnResourceStepPre` and then assert we've already seen it in
`OnResourceStepPre` and `OnResourceOutputs` handlers.
This means that shouldShow is now a pure function and makes it easier
to move the decision on if we should print information about a step
out of the engine and into the CLI.
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