Add an option for fallback to additional lexicon if not found

[balmas]

Currently if a language has multiple lexicons, it's possible to select more than one for lookup for both short definitions and full definitions. But this always adds the definitions from each lexicon that is selected.

A more useful scenario would be to support a "first-found" lookup, so that if a definition isn't found in the preferred lexicon, we search the others until we find it.

This would be a more scalable way to address issues such as alpheios-project/mjm#2

[balmas]

@kirlat I'm thinking about what we can do for the incremental for this but this is also definitely a use case we need to support in the lexical query refactoring.