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

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.