file-menu:revert-callback in frame:editor-mixin

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

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

  Informs the user that this action is not undoable and, if they still want to continue, calls revert.