Added a plugin mechanism for allowing plugin components to appear in the Tools or Visualization menus. Plugins just need to implement GUICHooser.GUIChooserMenuPlugin and have a PluginManager.props file that has an entry like so:

weka.gui.GUIChooser.GUIChooserMenuPlugin=<fully qualified name of implementing plugin class>

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10811 e0a1b77d-ad91-4216-81b1-defd5f83fa92