Displaying explicit and implicit required fields

[jsheunis]

The psyinf json-editor theme (see 779afed) allows required fields to be displayed explicitly to the user, as opposed to the user only finding out on submission/validation.

Firstly, required fields that are direct properties of a schema are easily annotated.

Then, some properties of a schema might be:

objects that have required and non-required properties themselves
arrays that may have a minItems requirement
array item fields that may be required or not
Currently, only the direct required properties of a schema, and a property that is an object which itself has required properties, can be annotated as required. The rest are still to be figured out.