Respect order of field groups

Added a failing test

This is working for the fields in the _field_selector container. For the usual field widgets, it is setting weights correctly, but these are not being rendered in the right order. I'm unsure why that is.

EntityFormDisplay::processForm seems to be the culprit. Setting to Needs Review for feedback.

I've updated the MR so we're not ever setting decimal weights in the original form. Instead we use a decimal value in an intermediary array, sort that array, and then set the weights in the original array. The new weights should be sequential starting from 0.