NEW : prepared for Slicer Extension

- in CMakeList.txt : if built as a SLicer Extension, set EXTENTION_VARIABLES, set SUPERBUILD to OFF, and look for SLICER and SLICER_EXTENSION_PACK
- in src/CMakeList.txt : if built as a Slicer Extension, do not redefine install destinations
- in vtkPVPostFilter : got rid of vtkStrString and used std::string instead