CSSTUDIO-2889 Set 'changeListenerAdded' to 'false' when the change listener has finished running.

Author: abrahamwolk

core/ui/src/main/java/org/phoebus/ui/docking/DockPane.java

@@ -466,6 +466,7 @@ public void changed(ObservableValue observableValue, Object oldValue, Object new
                                 f.accept((Scene) newValue);
                             }
                             sceneProperty().removeListener(this);
+                            changeListenerAdded = false;
                         }
                     }
                 };