Use ChildInterface

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony-cmf/symfony-cmf/issues/188
| License       | MIT

See https://github.com/symfony-cmf/symfony-cmf/issues/188.

I deprecated the ``getParent``/``setParent`` methods in favor of the ``ChildInterface`` ones. If it's ok, I'll do the same for the other bundles.