Qt Designer: Write out idbased-attribute only when set

Otherwise, all new forms saved will contain
<ui version="4.0" idbasedtr="false"> which causes unnecessary
changes and upsets older versions of uic.

Amends 85624e87

.

Task-number: QTBUG-34610
Change-Id: Ie688c5b9d1075bca96374d98b682dbbfb8bf7eca
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>