Add N_() to allow adding translation keys without direct gettext() call.

scripts/translation-update.sh

@@ -14,6 +14,7 @@ updatepot()
  OPTS=$OPTS' --package-version='$VER
  OPTS=$OPTS' --default-domain=openscad'
  OPTS=$OPTS' --keyword=_'
+ OPTS=$OPTS' --keyword=N_'
  OPTS=$OPTS' --files-from=./po/POTFILES'
  cmd="${GETTEXT_PATH}xgettext "$OPTS' -o ./po/openscad.pot'
  echo $cmd

src/qtgettext.h

@@ -14,6 +14,8 @@
 #include <QString>
 #include "printutils.h"
 
+#define N_(String) String
+
 inline QString _( const char *msgid, int category )
 {
 	Q_UNUSED( category );