UI for configuring the display

[wking]

This will allow users to toggle between collapsed (#9) and expanded (#8) views, whether closed issues should be walked (#13), whether there should be a ceiling/floor hop limit (#17), whether alternative dependency sources should be respected (#20), etc. I think we probably want a config view which pushes this information up into App, which in turn pushes it down into DepGraphView. That would give us plenty of space for the settings and explanations of what they do without cluttering the dep-graph itself. Things that users were expected to toggle frequently (e.g. collapsed/expanded) could be hot-keyed at the App or DepGraph level.

---

depends on #66