Localize nsl messages
Review Request #129080 - Created Oct. 1, 2016 and submitted
LabPlot GUI shows partially untranslated messages from nsl library (those that are not extracted in other parts of the application).
This patch allows messages extraction with no harm to nsl portability (defines i18n() as a dummy function).
Tested. Messages can be successfully extracted. Compiles, all messages are shown translated.