Refactored the picker so that it is more generic
The picker now also works in the case the scope_path of the entity
should not be filtered on. Also, it is now easier to add new
functionality due to the fact that there is a seperate Field type
to handle it now.
Patch by: Sverre Rabbelier
Added a ReferenceField widget
Also removed the unused ReadOnlyBool widget (as we now have a
properlyworking ReadOnly widget).
Patch by: Sverre Rabbelier