Expose vtk object for access from python

For testing code written in python, you sometimes need to get access
to the vtk render window and related structures to get pixel data or
other related data.  Making the methods invokable provides this
functionality.

Libs/Visualization/VTK/Widgets/ctkVTKAbstractView.h

@@ -67,21 +67,21 @@ public slots:
 
 public:
   /// Get underlying RenderWindow
-  vtkRenderWindow* renderWindow()const;
+  Q_INVOKABLE vtkRenderWindow* renderWindow()const;
 
   /// Set/Get window interactor
-  vtkRenderWindowInteractor* interactor()const;
+  Q_INVOKABLE vtkRenderWindowInteractor* interactor()const;
   virtual void setInteractor(vtkRenderWindowInteractor* interactor);
 
   /// Get current interactor style
-  vtkInteractorObserver* interactorStyle()const;
+  Q_INVOKABLE vtkInteractorObserver* interactorStyle()const;
 
   /// Get corner annotation \a text
   QString cornerAnnotationText() const;
-  vtkCornerAnnotation* cornerAnnotation()const;
+  Q_INVOKABLE vtkCornerAnnotation* cornerAnnotation()const;
 
   /// Get the underlying QVTKWidget
-  QVTKWidget * VTKWidget() const;
+  Q_INVOKABLE QVTKWidget * VTKWidget() const;
 
   /// Get background color
   virtual QColor backgroundColor() const = 0;