Only call QA::updateAccessibility() if the "control owner" is a QWidget

QGraphicsItems are not accessible, and calling updateAccessibility()
with a QGraphicsTextItem caused the accessibility framework to try
to load a plugin for QGraphicsTextItem. Since that was not found, it
dumped this on the console:
"Cannot create accessible interface for object: QGraphicsTextItem"

Task-number: QTBUG-29720

Change-Id: I4aaa08ee6e97f03617fa08da0c6ef35992efdebf
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>