Small fix for the ScaledSliderRepresentation.java to allow disabling while keeping is selectable

Replaced the function to disabled the Node element to disable the slider
in it since disabling the Node automatically make it non clickable
anymore

Author: marcoleorato-ess

app/display/representation-javafx/src/main/java/org/csstudio/display/builder/representation/javafx/widgets/ScaledSliderRepresentation.java

@@ -408,7 +408,7 @@ public void updateChanges()
     {
         super.updateChanges();
         if (dirty_enablement.checkAndClear())
-            jfx_node.setDisable(! enabled);
+            slider.setDisable(!enabled);
         if (dirty_layout.checkAndClear())
         {
             final boolean horizontal = model_widget.propHorizontal().getValue();