Add generated picard/countries.py
Replace the release countries in const.py with import from countries.py.
Use ungettext_countries() for translating country names.
Add po/countries/*.po from transifex
Build, install and use all available locales

Drop the use of UI_LANGUAGES in setup.py which was restricting available translations,
even though translation was available from transifex.

Locales not in UI_LANGUAGES cannot be selected from Picard Advanced UI option,
but will be used if it is set to system default.
Add command get_po_files to setup.py

It runs tx pull with needed options.
Translations with less than 5% translated are ignored.
Use a constant for common options of tx pull
Build countries.py string as unicode and encode as utf-8 at write time

It was suggested by @mwiencek
Explicitly escape single quotes and use named format