SeoMetadata as mapping instead of listener work to assoc

| Q             | A
| ------------- | ---
| Bug fix?      | [yes|no]
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #149 
| License       | MIT
| Doc PR        | need to be refactored
This isn't nearly ready but it is a solution to show.

The following tasks need to be done:

* [x] persisistence for phpcr-odm
* [ ] persistence for orm
* [ ] just key, values instead of `Extra` object
* [ ] form type for those key-value-pairs
* [ ] configuration @WouterJ ?
* [ ] clearing CS

but this can give an overview how it can look like.