LC_ActionLayersExport: Mark messages for translation

[thep]

In librecad/src/actions/lc_actionlayersexport.cpp, a translated message in LC_ActionLayersExport::trigger(), "No %1 layers found", acquires the argument for the %1 placeholder from untranslated strings:

        QString exportModeString = (exportMode == SelectedMode) ? "selected" : "visible";

which should also be marked for translation.