Store changed status of the QAxWidget's "control" property correctly.

Handle the case of it being PropertySheetStringValue.

Task-number: QTBUG-37602

Change-Id: Iff54d6409ffabf48406fcfec96289848a42b3e55
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>
(cherry picked from qttools/9ee8213c7becfdf906ca781df4895e18b273e023)