GroupedDropdownField was showing empty values in Readonly mode due to not correctly handling the hierarchical source array.
Uses flattened source now in GroupedDropdownField->performReadonlyTransformation()
These bugs were never present in SS4 as the relevant code had a
substantial rewrite at the same time they were introduced in SS3.
In SS3, test C still fails.
Fixes#4705Fixes#4987Fixes#4793
Steve Boyd
Maxime Rainville
Martin Heise
Daniel Hensby
Sam Minnee
Damian Mooyman