Fix "additionalProperties" support

[handrews]

It is currently implemented mostly like "properties" but with the additional ability to merge into the results of an adjacent allOf/oneOf/anyOf. However, per the spec, "properties" already behaves this way (adjacent schema keywords are effectively an additional branch of an implicit allOf anyway). And "additionalProperties" takes a single schema, not a set of names.

This is a json-schema-example-loader issue in object-definition.js

[handrews]

Part of this is also in json-schema-example-loader's example-data-extractor.js

[handrews]

I took the "theme" label off because we were not displaying "additionalProperties" before, so the part of this that is a bug fix is just in json-schema-example-loader.

Adding "additionalProperties" to the theme can be a separate, lower priority issue.

[handrews]

This should have been closed with the example-loader fix back in May, I believe. Although the current bootstrap theme cannot make use of it, that's no worse than it was.