Highlight the curve belonging to the legend you click on. (#9259)

* Highlight the curve belonging to the legend you click on.
* Make sure legend curve order remains the same when changing hilited curve

ApplicationLibCode/UserInterface/RiuPlotWidget.h

@@ -188,6 +188,9 @@ public:
 
     virtual void updateZoomDependentCurveProperties();
 
+signals:
+    void curveOrderNeedsUpdate();
+
 protected:
     void updateOverlayFrameLayout();