Add a "Studio Description" field on all features

All features should hold a "description" field for documentation within Studio (like the "documentation" field on automation chains).

There are 2 cases in Studio:

• studio_description_at_creation.png: the element created has a creation screen (document creation), so the description can be directly filled in this first step.
• studio_description.png: the element created has no document screen, the description has to be filled in the global screen.
  In both cases, user can edit an modify the description in the global screen through the Edit link.