ShortenPoFiles(sourceSet: I18nSourceSet)
Shortens the *.po files in the given source set.
This means all occurences of the source code locations of the translated strings are stripped, as well as the last translator of the *.po file. Also some fields in the file header are filled out (package name, copyright holder and a descriptive title).
This task should be run after downloading fresh translations from Transifex (e.g. with TransifexDownload).