Add FieldDescriptionInterface

[VincentLanglet]

Subject

Related to
#234
#233

I'm trying here

• To move a (smaller) FieldDescriptionInterface from AdminBundle to DatagridBundle in order to use it.
• To stop duplicating Datagrid/DatagridInterface/Filter/FilterInterface between AdminBundle and DatagridBundle. In the next major, AdminBundle will use the DatagridBundle classes.

I'm also trying to stop to be Doctrine-related in the DatagridBundle and the AdminBundle

• By exposing FieldMappingInterface and AssociationMappingInterface instead of array with specific key.
• By exposing AssociationMappingInterface constants instead of ClassMetadata ones.
  DoctrineORMAdminBundle will then do the mapping between our classes and the doctrine ones.

Steps are

• Merge into master
• Require datagrid-bundle@dev-master in admin-bundle@dev-master
• Use/Extends the Datagrid interfaces in the AdminBundle
• Remove some of the AdminBundle interface

Changelog

No changelog since it's on master, but I added an upgrade note.

TODO

• Add methods from https://github.com/sonata-project/SonataDatagridBundle/pull/233/files#diff-a510508a0ff7315f721b5343a392c9c0
• Rename entityJoin (?)

[VincentLanglet]

@core23 @phansys @greg0ire WDYT about this solution ?

[SonataCI]

Could you please rebase your PR and fix merge conflicts?