renamed MenuNodeCommonAdmin to AbstractMenuNodeAdmin and changed it to abstract

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #117
| License       | MIT
| Doc PR        | no

The follow up to https://github.com/symfony-cmf/MenuBundle/commit/1244d76a4f7a92dcef27d3912c56782985880530 as promised