Qt Designer: Refactor FormWindowManager::getUnsortedLayoutsToBeBroken()

Change the return type from a QMap<QWidget *,bool> to a QSet<QWidget *>
to fix a clang-tidy warning that a QHash should be used. The bool is
redundant, though.

Change-Id: Ib0bced9166cb6050e48195851abd6cb2b5109537
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>