Work out a concept for dict setting completions

[FichteFoll]

Currently, we just ignore dicts when offering value completions, because

1. we don't know how to handle them in most situations, especially when they occur as secondary values inside a list,
2. our architecture can't handle unhashable type (and we have special handling for lists)

Completions currently originate from either the default value, parsing comments and indirectly from list items of a default list value. Currently it seems only completing the default value it sensible.

Related: #156, 2fba8f5