Several items of work related to adding a manual creation of occluders

option to the osgoccluder demo.

First the osgGA classes have been generalised a little to better support
new GUIEventHandler subclasses.

Second osgGLUT has a new option for registering a user defined event handler,
the allows the application to add externally extra options to the osgGLUT viewer.

Third, the osgoccluder demo now works in two modes. The original create
four occluder sides to wrap an loaded model, the second an interactive
mode which allows the users to add occluders to the model by pointing
to points and pressing 'a' to add a point, the 'e' to end the occluder
polygon, polygons must be convex, planer and be defined in an anitclockwise
order.  To start the osgoccluder in this mode one simple runs it with the
'-c' option.  Run osgoccluder with no parameter to get a list of options.

include/osgGA/GUIEventHandlerVisitor

@@ -9,6 +9,7 @@ namespace osgGA{
 
 // Some forward declarations
 class GUIActionAdapter;
+class GUIEventHandler;
 class CompositeGUIEventHandler;
 class CameraManipulator;
 class StateSetManipulator;
@@ -26,6 +27,7 @@ class OSGGA_EXPORT GUIEventHandlerVisitor
 {
     public:
 
+        virtual void visit(GUIEventHandler&) {}
         virtual void visit(CompositeGUIEventHandler&);
         virtual void visit(CameraManipulator&) {};
         virtual void visit(StateSetManipulator&) {};