This widget makes it easy to use the VTK rendering framework in external
window managers and/or external applications that do not require VTK to draw its
own window. ExternalVTKWidget uses the vtkExternalOpenGLRenderWindow and
vtkExternalOpenGLRenderer classes to use the OpenGL context managed by the
external application.

Change-Id: I629ee93d427e0d0c791748c010b597b6e327f2a9