Refactor layout and widget system

• layouts/widgets model refactoring (needs persistence testing first) to:

• address some concepts that are not available in studio:

• widgets do not have the same properties depending on the mode (edit or
  view)
• preferred widget type mapping for a field: should not be forced on
  widget types that do not map a field (summary widgets for instance).
• layout rows (or columns) now also need some configuration for listing layouts

• use new features in nuxeo:

• new packaged widget types
• decorative widgets
• widgets with more configuration for rendering for instance

• map existing widgets types in nuxeo to Studio:

• make it possible for a live Nuxeo to export widget types
  configuration for Studio
• make it possible for Studio to use this configuration file to present
  available widget types

This would be consistent with system currently used for action, where
actions/operations annotations make it possible to retrieve a json file
that will be used by Studio.

This will open opportunities to improve existing widgets (there are a few
bugfixes about it), as well as make listing widgets available in studio
for folderish documents.