AbstractSingleSelect API

I think it would be a good idea to change AbstractListing.java methods setItemCaptionGenerator and getItemCaptionGenerator to have public access.

Also, I think it would be a good idea to have setEmptySelectionAllowed
and setEmptySelectionCaption added to AbstractSingleSelect this way we could write generic code that doesn’t care if the actual implementation is a ComboBox, NativeSelect or RadioButtonGroup.

This was possible in Vaadin 7 with AbstractSelect

I made GitHub ticket about this: