Support validation of profiles extending other profiles

**Is your feature request related to a problem? Please describe.**
New profile shave to be copied to extend existing profiles. But the DTR allows defining an "extends" attribute, which we could consider (we most likely do not do it yet).

**Describe the solution you'd like**
Validation should consider the extends attribute and merge the definitions (preferred) or validate accordingly.

**Describe alternatives you've considered**
Let people copy profiles, as it is currently the case.

**Additional context**

- "Overriding" properties is then possible to some degree (e.g. change repeatability).
- Could also be implemented for types, if the DTR supports it.

Start by locating profile validation and the DTR handling for the "extends" attribute. Check how profile definitions are currently copied and how overriding properties such as repeatability should behave; done means inherited definitions are merged or otherwise validated, with tests covering the behavior.

Written by the indexing model from the issue text.