Make it possible to set FontAwesome icons after the label for action links

By using org.apache.isis.applib.annotation.ActionLayout#cssClassFa() one can set a custom FontAwesome icon for an action link.
The small issue that I see in ToDo App is that the icon is used always as a prefix for the link's label.
Sometimes it may make more sense to use if as a suffix, e.g. for the "Next" link in "Priority" section. See screenshot.