Support additional properties

[acka47]

I would like to add some extra properties to a JSON Schema, that are not initially shown in the editor but if the user clicks on something like "show additional properties". One question is how to mark those in the JSON schema. (We should not handle all those properties as additional that are not listed as required.Taking a short look, I did not find anything.

[acka47]

Background: I would like to write a schema that covers all/most of the LRMI properties in schema.org while distinguishing some core properties for quick and simple description vs. some additional information with regard to educational use, audience etc.