[entity] Add a cw_related_rqlst method returning the RQL select node of cw_related method This is useful if one wants to modify the query before execution. A bit a PEP8 style fix in tests along the way. Related to #12306543.

.. _cwprops:

The property mecanism
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.. XXX CWProperty and co


Property API
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.. XXX feed me

Registering and using your own property
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.. XXX feed me