Provide some way of visualising the dependency graph of a given element

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/705)
In GitLab by [[Gitlab user @coldtom]](https://gitlab.com/coldtom) on Oct 11, 2018, 17:07

## Background

It would be cool if BuildStream had a way to visualise the dependency graph of a given element, especially if one could look at the graphs above and below separately. In terms of a valid use case, I've often wanted this feature when deciding whether to test changes by building locally or putting them through CI, as some elements in freedesktop-sdk take too long to feasibly build locally. This feature would allow users to do something like this much easier.

Start by reviewing the original GitLab issue and the described use case around selecting elements for local builds or CI. Define what visualising a given element's dependency graph means, including separate graphs above and below it; done means users can inspect both views.

Written by the indexing model from the issue text.