migrate from itstool to msgfmt for creating translated XML
gettext upstream has supported this for a very long time, and using it means we don't need to jump through hoops to run ninja inside ninja in order to generate the .mo files for itstool.
Although meson 0.60.0 added support for real targets for the built .mo files, this changed the rules for output filenames, breaking the script. But msgfmt when used directly does not care.
Fixes #170 (closed)