Remove the libintl usage
Switch RKMessageCatalog to KLocalizedString, as it can use it perfectly to translate the messages of plugins from arbitrary locations.
Always translate the messages in the R backend, no matter whether R was built with NLS support, as they will be shown to the user with a translated rkward. Switch to ki18n for those messages as well.
With these changes, libintl is no more used directly, and thus its usage is dropped.
Edited by Pino Toscano