Qt Designer: Refactor WidgetFactory::isPassiveInteractor()

Factor out a helper routine to check on the widget so that the
caching pattern becomes clearer. Change the heap-allocated
QPointer<QWidget> to a static variable.

Change-Id: I5dfc5a9ebc791dbd2e13a9cbde72f77c3e2c6104
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>