NEW nogui example now successful + nogui test + safeguard around all Application::getViewer(Viewer*)

The nogui application is an example of an single independent program that
uses CamiTK to load an image, apply a CamiTK action and save the resulting image to file.
Safeguards around Application::getViewer(Viewer*) avoid possible crashes while
providing error messages.
Note: the nogui success is now automatically tested as well