Sofa for windows

Multiple things are done here to allow SOFA tu build on windows:
 - Add check for Qt > 4.8.3 as Sofa needs it
 - Add find_lib and FindSOFA for the CLIs that will need it
 - Add external project step for windows that downloads the SOFA
dependencies in sofa sources. Although this is ugly, it's the solution that
yelds the least overhead while still making the build of bender a one-click
step.