This method is called when the replace-and-find-again menu-item of the edit-menu menu is selected. If edit-menu:replace-and-find-again is bound to #f instead of a procedure, the replace-and-find-again menu item will not be created.
Returns #t, and if the selected text matches the current text in the find box, replaces it with the contents of the replace box and searches for the next occurance of the text in the find box.