[entities] cw_rest_attr_info() should only consider required attributes (closes #3766717)
This prevents CW from choosing unique but non-required attributes. None/NULL is
a poor choice for RESTful URIs.
.. _cwprops:
The property mecanism
---------------------
.. XXX CWProperty and co
Property API
~~~~~~~~~~~~
.. XXX feed me
Registering and using your own property
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. XXX feed me