Fix container widget on 5.6 when hiding subwidgets

The contained widget on 5.6 uses properties set on the container, with an index, to declare if its subwigets should have a surrounding form or not.

But when hiding one of the subwidgets, index can change, so form around widget may be buggy.

This problem is not present on 5.8 thanks to widget controls, held by subwidgets and checked by the parent container widget, so a specific fix is needed on 5.6 branch for this use case.