use css to hide overlong labels in tree

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/pull/223
| License       | MIT
| Doc PR        | -