[webrequest] Lazy-load search_state
- The update_search_state function will hit the session data only if __mode is
present in the form.
- set_search_state keep the same semantics
- Direct access to search_state in the session data is changed by an access to
the new property.
Closes #4875761
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