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.

Thanks to criezy for his suggestion on this approach.
Merge pull request #378 from lordhoto/pluggable-engines-translations

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