[devtools] bases for instrumenting / debugging standard propagation hooks
* a class that helps writing command to check propagation, detecting standard
errors and picture the propagation as a graph
* a class to instrument sets used to configure standard propagation hooks
closes #3171980
usr/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py