[selectors, facet] make is_instance and other EClassSelector subclass gather entity classes from 'select' and 'filtered_variable' context information. Closes #1796732 This allow for instance to use the filter form on rqlst where the filtered variable is not selected

sync_schema_props_perms('cw_source', syncprops=False)
add_attribute('CWSource', 'synchronizing')
if schema['BigInt'].eid is None:
    add_entity_type('BigInt')