Fix bug where undoing+redoing AddWire multiple times crashes

Connector.h

@@ -9,7 +9,7 @@ class Part;
 class Wire;
 class Scene;
 
-class Connector : public QGraphicsItem
+class Connector : private QGraphicsItem
 {
 public:
     friend class Scene;
@@ -24,6 +24,8 @@ public:
 
     Connector(Scene* scene, Part *parentPart, ConnectorType::ConnectorType side);
 
+    Part* parentPart();
+
     QRectF boundingRect() const override; // For drawing
     QPainterPath shape() const override; // For selection ("Hitbox")
     void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override;