Sub-schema could only be selected when they have previously be created. Now we allow for creating sub-schemata in place:
- the system suggests also sub-schema which do not yet exist
- I can confirm the suggested sub-schema and this schema is created "on the fly"
- The editor switches to the auto-created schema, which does initially not have any attribute paths, and allows me for editing this schema
- for better overview breadcrumbs show me which (nested) schema I am currently editing