Memory leak with FlowScene

[Quincunx271]

Currently, if you aren't careful, it's really easy to leak memory with FlowScene. ~FlowScene() isn't called by default. You have to write scene->setParent(parentWidget), otherwise it's not automatically deleted.

A possible fix would be to modify the constructor of FlowScene to something like this:

  FlowScene(std::shared_ptr<DataModelRegistry> registry =
              std::make_shared<DataModelRegistry>(),
            QWidget* parent = nullptr);

FlowScene::
FlowScene(std::shared_ptr<DataModelRegistry> registry, QWidget* parent)
  : _registry(registry)
{
  setItemIndexMethod(QGraphicsScene::NoIndex);
  setParent(parent);
}

[paceholder]

@Quincunx271 #155