[FEATURE] Refactored webspace collection array serialization

- Moved responsability for converting objects to arrays to the objects
  themeselves to enable reuse elsewhere.
- Added unit tests for all webspace classes which implement `toArray`

This is a prerequisite to including information on the objects in this PR in the debug toolbar.

This information should increase developer productivity.

- [x] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | 
| BC Breaks     | no
| Doc           | 

We should return the entire tree here, and not just the first children