mki18n.py: generate messages.po (contains English strings in msgstr)

This po file is necessary because Transifex translation platform
automatically can only import PO files. Usually messages.pot is
enough.

<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot URI_location="LOCAL://">
  <TargetType/>
  <Libraries Enable_SVGUI_Library="false"/>
</BeremizRoot>