[entity] add an official ``cw_etype`` property for entity
This property holds the CWEtype id of the entity. This information was previously
available through __regid__ but this is an ugly public API. So we have a new
clean public API. The ``dc_type()`` method could not be used as it is
translated.
(closes #2793792)
doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc