Add convenience methods for creating vtk objects.

Stuff like lines, spheres, circles and bounds for volumes. Anything
useful that can be displayed in a vtkRenderer.
Make use of the vtkDrawing functions for drawing objects with the render widget.
Make use of the vtkDrawing functions for the TwoStepPicker.
Make use of the vtkDrawing functions for Landmark class.

Also provide a way of scaling the interface elements. This can be used
to scale the objects according to the size of the current volume.
Scale the landmarks according to the size of the image data.
Adjust the layout of the transformation tools for better spacing.