New backend() method in view API to access underlying implementation
adds a vtkImageView3d as a variable of the medVtkViewBackend, corrects an error in implementation

Conflicts:
	src-plugins/variationalSegmentation/varSegToolBox.cpp
adds the vtkRenderWindow to the backend
Merge pull request #118 from fvichot/new-view-backend-api

New backend() method in view API to access underlying implementation