getValue() type inferenz von weiteren objekten unterstützen

**Feature description / Feature Beschreibung**
z.b.
```
$art = new rex_article_content($parentId);
$parentId = $art->getValue('parent_id');
```

- [ ] rex_article_content
- [x] rex_article
- [x] rex_category
- [x] rex_article_slice
- [ ] rex_clang
- [x] rex_user
- [ ] rex_login
- [ ] rex_form ?
- [ ] rex_list ?
- [ ] yform XX ?

Start by locating the getValue() type-inference implementation and compare how rex_article, rex_category, rex_article_slice, and rex_user are currently handled. Determine the intended support for rex_article_content, rex_clang, rex_login, rex_form, rex_list, and yform, then verify that getValue() infers the expected types for the supported objects.

Written by the indexing model from the issue text.