The GenericPropertiesCreator.props must contain the following lines in order
to display the Main extensions:

# List of extensions for the main GUI, package "weka.gui" is not allowed
weka.gui.MainMenuExtension=\
  wekaexamples.gui.extensions

See also wiki article "Extensions for Weka's main GUI":
  http://weka.wiki.sourceforge.net/Extensions+for+Weka%27s+main+GUI
