Implemented a fix for the custom sort not seeing relations after loading a model

src/main/java/nl/andrewlalis/erme/model/MappingModel.java

@@ -28,7 +28,6 @@ public class MappingModel implements Serializable, Viewable {
 	public MappingModel() {
 		this.relations = new HashSet<>();
 		this.changeListeners = new HashSet<>();
-		this.addChangeListener(OrderableListPanel.getInstance());
 	}
 
 	public void addRelation(Relation r) {

src/main/java/nl/andrewlalis/erme/view/DiagramPanel.java

@@ -64,6 +64,7 @@ public class DiagramPanel extends JPanel implements ModelChangeListener {
 		this.addMouseListener(listener);
 		this.addMouseMotionListener(listener);
 		this.updateActionModels();
+		newModel.addChangeListener(OrderableListPanel.getInstance());
 		this.centerModel();
 		this.repaint();
 	}