cubicweb #1796732 in some cases we want is_instance to retrieve class from rql syntax tree [resolved]

eg if we want facets for a rql where the filtered variable isn't selected, but we've class information in the rql syntax tree

done in3.13.1
load left0.000
closed by#838a0c910bb5 [selectors, facet] make is_instance and other EClassSelector subclass gather entity classes from 'select' and 'filtered_variable' context information. Closes #1796732