QmlDesigner: Prevent freezing while doing drag&drop on some Windows

systems

QWidget::grabMouse() might freeze if there are e.g. misbehaving virus
scanners installed. This is why e.g. qdockwidget.cpp doesn't use
grabMouse(), but QWidgetPrivate::grabMouseWhileInWindow().

As a hot fix we do the very same now in creator, at the cost of
including the private header qwidget_p.h.

Task-number: BAUHAUS-772
Task-number: QTBUG-11301