Icons / AwesomeIconStack don't work with scene builder

Since the default constructors of these classes are private, scene builder is incapable of correclty importing them and won't load the FXML file.

If the constructors are made public, it is very easy to import the fontawesomefx jar into scene builder and have access to the Icon class without issues. When left private, SceneBuilder simply won't load the fxml because of exceptions while reading the file.