Added missing menu properties

KnpMenu item is now fully mapped to PHPCR.

Also have briefly documented the purpose of the properties in the class, see issue: https://github.com/symfony-cmf/symfony-cmf-docs/pull/157 for documentation.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | no