Display a label instead of the id for Directory/Document/User/Group metadata in the CSV export

Currently, the CSV export contains the raw data for any kind of field which could make their use or reading quite difficult in a non technical environment.

The purpose of this ticket is to handle the specific metadata entities :

• Directory entity should display the vocabulary label instead of the id. It means that at least 3 cases must be managed:
  • localized vocabulary => the export should contained the localized label
  • dbl10n vocabulary => the export should contain the label for the current language
  • parent/hierarchical vocabulary => the export should display the label for parent and child value
• Document entity should display the title
• User entity should display first name and last name (+ maybe the email address)
• Group entitry should display the group label

There are maybe other cases with different entities. We may be interested in providing a way to describe how entities should be exported (as it was the case for JSF widgets)