Display whether an element builds artifacts when showing if it's buildable

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1137)
In GitLab by [[Gitlab user @coldtom]](https://gitlab.com/coldtom) on Sep 16, 2019, 10:28

## Background

[//]: # (Provide a background or the root/source that justifies this task or action.)
Since !1593 merged we have an abstraction to tell us if an element actually produces an artifact. This means that we could now add some UI to display whether an element will produce an artifact when built.

## Task description

[//]: # (Short summary of the action to be executed)

* [ ] Display whether an element produces an artifact when built, when we show whether elements are buildable

## Acceptance Criteria

[//]: # (Acceptance criteria should follow the S.M.A.R.T. principle https://en.wikipedia.org/wiki/SMART_criteria )

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[//]: # (To review information about possible relevant labels for this issue please view the list of labels: https://gitlab.com/BuildStream/buildstream/labels)

Review the artifact-producing abstraction introduced by !1593 and locate the existing UI that reports whether elements are buildable. Add the artifact-production status to that display, then verify that the information is shown for relevant elements.

Written by the indexing model from the issue text.