Register UI schemas for definitions

**Feature Request**
Some renderers will look for registered ui schemas to render nested elements. It would be useful if additional ui schemas could not only be registered on feature paths like `myarray/items/properties/nestedarray/items` but on definitions in JSON schema.

**Solution ideas**
JSON Forms should be able to use ui schemas registered on definitions paths. For this it could for example keep the unresolved schemas around or build up a map of paths to resolved (sub)schemas.