Instead generate it as a pre-build step. It can't be generated as part
of the CMake build process because MSVC changes the PATH so that another
convert.exe conflicts with imagemagick's.
It requires Inkscape and ImageMagick to be installed. This does not
replace the existing icon.ico file because of issues integrating the
script into the CI build system, but at least the process is now
documented and reproducible. Closes#713.