Support RetroLab-like applications

[bollwyvl]

Elevator Pitch

Doing as few special case code changes as possible, support launching in a applications with fewer core Lab features enable.

Motivation

RetroLab represents both an example, limited application, as well as a concrete, first-party-supported application which will likely stay up-to-date with Lab core. It is likely to be adopted by more conservative Hub deployments as a default application rather than throwing full-blown Lab at potentially first-time Jupyter users.

Design Ideas

• add to IFileBrowser toolbar
  • maybe add a dialog for "custom diagram" (buh)
• move as many required IWhateverToken to optional
• do some CSS detection to consume all of the gutter space
• replicate some control surfaces in more places (e.g. IMainMenu)
• add to Robot tests
• update docs