Helpers to override `buttonClicked` added.

These changes were made to primarily to accommodate for the cyclops-stimulator. Since buttonClicked manipulates the DataViewport via AccessClass, a sub-class of VisualizerEditor cannot functionally override the method.
Hence:

* Added protected methods which manipulate DataViewport (via AccessClass)
* moved the SelectorButtons into protected area, essential!
* All of the additions are essential to the cyclops-stimulator.
* There is NO change in the public interface.

Now, my buttonClicked method can access the SelectorButtons, manipulate (anything about!!) DataViewport.
Please remove the (new) comments from VisualizerEditor::buttonClicked (.cpp), they are not needed anymore.