Autocomplete or dropdown for filling in cells where choices are constrained

[KevinDCarlson]

The interface should surface implicit constraints, for instance if I'm constructing a new aspect then I should get a dropdown of objects to choose from rather than having to re-type the name. Possibly autocomplete too/instead, especially for long names. This will be more and more important for more complicated schemas; if we make it to something like data migrations it should be a killer app (and require pretty substantial logic to correctly compute.)

[KevinDCarlson]

This is probably subsumed by #22