Add scripts and CI job for building Windows package

The scripts are based the former “dynare-build” project. They have been
overhauled and simplified.

Building a Windows package (both installer and zip archive) is as easy as
running “make -C windows” (provided the right Debian packages are installed,
use the “windows/install-packages.sh” script for that purpose).

The layout of MEX files for Octave in the package has been
changed (mex/octave/win32/ and mex/octave/win64/ instead of mex/octave32/ and
mex/octave/), for consistency with MATLAB MEX.