Pyside requires several tools that do not provide Python modules. They
therefore do not need to be build Python-version dependent and so we
move them out of `python-packages.nix`.
Furthermore, shiboken needs libxml2 and libxslt libraries but not their
Python bindings.
I had to update all the pyside programs, or freecad failed to build. I picked
the versions advertised in http://qt-project.org/wiki/PySideDownloads . The
rest I took for github latest releases.
