file-menu:save-as-callback in frame:editor-mixin

This method is called when the save-as menu-item of the file-menu menu is selected.

• (send a-frame:editor-mixin file-menu:save-as-callback item evt) -> void
     item : (instance (derived-from menu-item%))
     evt : (instance control-event%)

  Prompts the user for a file name and uses that filename to save the buffer. Calls save-as with no arguments.