Address known issue of commit 900fd73f,

> On moving a ComponentWidget, its attached PortWidgets should move along.

umbrello/widgets/componentwidget.{h,cpp}
- Add signal sigCompMoved(qreal diffX, qreal diffY)
- Reimplement UMLWidget::moveWidgetBy() for emitting sigCompMoved()

umbrello/widgets/portwidget.{h,cpp}
- Add slot slotCompMoved(qreal diffX, qreal diffY)
- In constructor, connect signal sigCompMoved() of the owning component's
  widget with own slot slotCompMoved()

CCBUG:115269