erp5_crm: Add support for deeper "use" resource hierachy.

Remove depth limitations on domain generator.
Make ListField item list generator present resource hierarchically (either
through indentation or through relative title-based paths, depending on
preference).
Also, share a bit of code between both.
erp5_property_sheets: Add Predicate/empty_criterion_valid_property .
erp5_core: Allow viewing/setting Predicate's empty_criterion_valid property.
erp5_web: Match no content by default on a new Web Section.

Before this change, an unconfigured Web Section would match all
available documents, which can be surprising (security would still apply,
of course).