Designer: Fix morphing of widgets in QSplitters.

They were not handled at all, children were just reparented
resulting in the morphed widgets being appended.

Task-number: QTBUG-31911
Change-Id: Idae8f66663416d514a2f3c299e58d726a3dd578a
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>
(cherry picked from qttools/d251ee9dcbd015f2acbe76231902b72163cd05a2)