BUG : remove GL2PS features for Slicer Extension

- CMakeLists.txt : create variable to define if Slicer Extension
- src/ShapePopulationQT.cxx : disgard "ExportTo" if Slicer Extension and display warning instead
- src/ShapePopulationQT.h : remove include GL2PS
BUG : add <vtkversion.h> to allow recognition of VTK 6 version