Correctly display promoted variable trees in the GUI

This fix just required an adjustment in get_attributes because a passthrough Variable is a PassthroughTrait, so you need to access the validation_trait in the metadata to get to the real trait.

Also, this exposed a problem in how we deepcopy a variable's value when we pass it. Containers like Variable Trees have a parent reference, and this must not be copied when passing. This problem was fixed.