private$session_bindingsCall a destroy method to remove observer and input from obsolete session which happens
when filter_panel_srv is called again in new FilteredData object.
Inputs are not stored directly in a field as they don't have destroy method. Instead, we
store callback destroy function for inputs which removes bindings from a session.
.finalize_session_bindings(self, private)NULL invisibly
slots of a R6 class