Gesture canceled via policy within QGraphicsScene is not removed from QGestureManager

• Ongoing gesture has target in QGraphicsScene (viewport, too)
• A second gesture starts, with cancel policy
• The previous gesture should be removed from both QGraphicsScenePrivate::gestureTargets and QGestureManager::m_activeGestures
  => it is not removed from m_activeGestures.

This causes every other gesture to fail to work properly whenever cancel policy is activated.