[3.0.x] .tx: add configuration to update po files

This is a configuration file for the tx command line tool to automatically pull updated translations from the transifex platform to the correct places.

After the pull, it is necessary to correct the headers manually and to perform "for f in *.po; do msgcat --width=79 -o "f" "f"; done" to achieve the intended formatting.

Perform the pull with "tx pull --use-git-timestamps --force"