Revert "Qt Designer: Fix scrolling of widget box."

This reverts commit 082efca145630db2d660d7241f7ae78d80f27e44.

Following the revert of 934f06220391eb0e0ebf66a2eb037f48adb4c43c
in qtbase.

Task-number: QTBUG-34953
Task-number: QTBUG-33906
Change-Id: Ib839ef4a3b43e136ab1ff298606d5b2945b2a51b
Reviewed-by: Thorbjørn Lindeijer <bjorn@lindeijer.nl>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>