ENH: Add ExternalVTKWidget

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: Ic1fa45daba6a340189961447e0bda961afee7196