make semantics of getPrevLinkable / getNextLinkable consistent

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  |yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #122
| License       | MIT
| Doc PR        | https://github.com/symfony-cmf/symfony-cmf-docs/pull/402

We started using isLinkable for getLinkableChildren, but not for getNext/PrevLinkable. This makes things consistent again.