Fixed the list filter layout

Fixed the broken list filter layout. Right now, not all fields are hidden when klicking the Filter heading. Now, all fields are hidden / shown when klicking the heading.

This fix is related to pull request https://github.com/sonata-project/SonataAdminBundle/pull/1241 and issue https://github.com/sonata-project/SonataAdminBundle/issues/1219.
![filter-open](https://f.cloud.github.com/assets/2149539/332648/29ac9fd4-9c35-11e2-8337-d270ac6d7eb2.png)