Use tab character, not \t in translatable string in .ny file...

... The \t escape is changed to tab by the XLisp reader, but that is not
standard Common Lisp behavior, which would just strip the \ character and leave
t.

xgettext follows standard behavior when making the .pot file, so using \t
caused a mismatch between the catalog and the string actually looked up,
resulting in a translation failure.

There was only one occurence of \t in translatable strings in all of plug-ins.
I left alone the occurrences in non-translatable strings.

plug-ins/adjustable-fade.ny

@@ -68,7 +68,7 @@ $control preset (_ "Handy Presets (override controls)") choice (("None" (_ "None
       ((or (> x 100)(> y 100))
         (throw 'err (format nil (_ "~adB values cannot be more than +100 dB.~%~%~
                                  Hint: 6 dB doubles the amplitude~%~
-                                 \t-6 dB halves the amplitude.") err))))))
+                                 	-6 dB halves the amplitude.") err))))))
 
 ;;; Select and apply fade
 (defun fade (sig type curve g0 g1)