[NEXUS-5491] Fix rendering of optional fieldset inside optional fieldsets

Data translation helper looked at deep nodes first and tried to
immediately expand fieldsets that were not rendered yet because the
containing fieldset was still collapsed.
Re-applying change lost in squash-o-matic branch

Original change in ad294cf9e52f8940229469c0ba641787c605552b,
extending FormPanel here causes nesting forms which chrome does not
like.