[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)

debian/tmp/usr/bin/cubicweb-ctl usr/bin/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/cwctl.py usr/lib/PY_VERSION/site-packages/cubicweb
debian/cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl