edit-menu:replace-and-find-again-callback in frame:searchable-mixin

This method is called when the replace-and-find-again menu-item of the edit-menu menu is selected.

• (send a-frame:searchable-mixin edit-menu:replace-and-find-again-callback boolean

  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 occurrence of the text in the find box.