help-menu:about-callback in frame:editor-mixin

This method is called when the about menu-item of the help-menu menu is selected.

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

  Calls message-box with a message welcoming the user to the application named by application:current-app-name