MultiPointHandlers (DragHandler, PinchHandler) don't emit the grabChanged signal

QQuickMultiPointHandler::onGrabChanged() overrides QQuickPointerHandler::onGrabChanged() and doesn't call it; whereas QQuickSinglePointHandler::onGrabChanged() does call it. This is inconsistent between handlers; and it could provide a useful "loophole" to see initial touchpoints at the time the gesture begins, since we don't provide API for that.