RFC: Pluggable Engines: I18N: Move specification of engine specific files to enginedir/POTFILES

This allows to keep the engines to specfiy the files for translation close to the engine sources itself.

This is based on the suggestion by criezy. I did not regenerate the translation files because even when using update-translations in master the files will get different from what we have right now. However, it seems input file ordering has an effect on the output order. So we might want to consider passing "-F" which sorts the output according to the location of the input files.