add removeChild and init children with Collection

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | maybe (if your code depends on children being a real array)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony-cmf/MenuBundle/issues/181
| License       | MIT
| Doc PR        | n.a.