The change is only relevant to development as all the call to python scripts is done
calling C++ os::python that invoques the appropriate python version.
The change is two fold, on one hand remove all the uses of /usr/bin/env for python.
On the other hand rename all the calls to python from python to python3 making it explicit
and being compliant with PEP 394 -- The "python" Command on Unix-Like Systems:
https://www.python.org/dev/peps/pep-0394/
Remove the sheebang from src/graphics/GraphicsConverter.cpp because it is not necessary.
Some small whitespace changes.
This target will update the gmo files if and only if the associated
po-file is modified. To determine whether a file is modified, 'git
status --porcelain' is used.
This follows up on commit 06782542.
