Use constant for batch, select and actions fields

[VincentLanglet]

Subject

BC

As you can see, we're using _actions for the name of actions, but batch and select for the others.
It could create a conflict if someone want to display an entity which has a select field.
So I recommend to prefix by _ in Next major.

Changelog

### Added
- Added constant `ListMapper::NAME_ACTIONS` to use instead of `_action`
- Added constant `ListMapper::NAME_SELECT` to use instead of `select`
- Added constant `ListMapper::NAME_BATCH` to use instead of `batch`

### Deprecated
- Relying on the code `_select` or `_batch` to determine if a field description is a `select` or a `batch` one. You should rely on the name `ListMapper::NAME_SELECT` or `ListMapper::NAME_BATCH` instead.

[franmomu]

I think we should either call it in singular or change it to _actions in next major version?

[VincentLanglet]

I added a comment.