[WIP] PIM-1703 Allow registration of mass edit actions specific to a datagrid

| Q                   | A
| ------------------- | ---
| Bug fix?            | no
| New feature?        | yes
| BC breaks?          | yes
| Tests pass?         | no
| Scenarios pass?     | no
| Checkstyle issues?  | ?
| Changelog updated?  | yes
| Fixed tickets       | PIM-1703
| Doc PR              | https://github.com/akeneo/pim-docs/pull/18

![myimage](https://cloud.githubusercontent.com/assets/668604/2741815/6cbbb53a-c6f5-11e3-88c3-d8ad302b9a4f.png)