Recognise inheritance between exported plugin classes

Exported plugin classes that inherit from other exported classes are treated as unrelated types by Objo: IsA against an exported base class returns False, and a derived plugin value cannot be assigned or passed to a parameter, variable or return position typed as its exported base. For component families built around an abstract exported base — for example a tree whose API hands back base-typed nodes — base-typed members are unusable, since no value can ever convert to the base type. Objo should recognise inheritance between exported plugin classes so IsA, assignment, parameters and returns follow the exported hierarchy.