fix the handling of the drracket:show-line-numbers? preference

so the line numbers change no matter who sets the preference

collects/drracket/private/main.rkt

@@ -338,15 +338,7 @@
    (λ (editor-panel)
      (make-check-box 'drracket:show-line-numbers?
                      (string-constant show-line-numbers)
-                     editor-panel
+                     editor-panel)
-                     (lambda (value)
-                       (define (drracket:frame? frame)
-                         (and (is-a? frame top-level-window<%>)
-                              (is-a? frame drracket:unit:frame%)))
-                       ;; is it a hack to use `get-top-level-windows' ?
-                       (define frames (filter drracket:frame? (get-top-level-windows)))
-                       (when (not (null? frames))
-                         (send (car frames) show-line-numbers! value))))
      
      ;; come back to this one.
      #;

collects/drracket/private/unit.rkt

@@ -1422,7 +1422,19 @@ module browser threading seems wrong.
                         (λ (l) (remq execute-warning-panel l)))
                  (send execute-warning-canvas set-message #f))])))
 
-        (define/public (show-line-numbers! show)
+        
+        ;; bind the proc to a field
+        ;; so it stays alive as long 
+        ;; as the frame stays alive
+        (define show-line-numbers-pref-fn
+          (let ([fn (lambda (pref value) (show-line-numbers! value))])
+            (preferences:add-callback
+             'drracket:show-line-numbers?
+             fn
+             #t)
+            fn))
+        
+        (define/private (show-line-numbers! show)
           (for ([tab tabs])
             (define text (send tab get-defs))
             (send text show-line-numbers! show))