cubicweb #1631210 i18n: split <lang>.po into auto_<lang>.po and extracted_<lang>.po [open]

When updating i18n translation catalogs, it's easy to get lost (even with emacs/vi dedicated edition modes) between schema-based msgids, cubicweb-generated msgids and extracted msgids. Moreover, msgids generated by cubicweb are sometimes not translated (because not always necessary), therefore making a final pass on application's untranslated messages can be painful.

IMO, splitting the po files into two (extracted and auto) po files would help.

done in<not specified>
load left1.000
closed by<not specified>