ENH: move superbuild files to specific subfolder

Superbuild/CMake/External_FLTK.cmake

Superbuild/CMake/External_GDAL.cmake

Superbuild/CMake/External_ITK.cmake

Superbuild/CMake/External_Ice.cmake

Superbuild/CMake/External_Monteverdi.cmake

Superbuild/CMake/External_Monteverdi2.cmake

Superbuild/CMake/External_OTB.cmake

Superbuild/CMake/External_OpenCV.cmake

Superbuild/CMake/External_OpenJpeg.cmake

Superbuild/CMake/External_OpenThreads.cmake

Superbuild/CMake/External_SQLite.cmake

Superbuild/CMake/External_TinyXML.cmake

Superbuild/CMake/External_boost.cmake

Superbuild/CMake/External_curl.cmake

Superbuild/CMake/External_expat.cmake

Superbuild/CMake/External_fftw.cmake

Superbuild/CMake/External_geos.cmake

Superbuild/CMake/External_geotiff.cmake

Superbuild/CMake/External_glew.cmake

Superbuild/CMake/External_glfw.cmake