* redirect system menu items only if an appropriate command exists
    (so that About & Quit continue to work)
  * invoke redirected commands directly rather than search through
    the menu items
  * process MMenuContributions too
  * install the CocoaUIHandler as an Addon rather than the hack
  * install a command listener to enable/disable the system menu items
    as commands come and go
  * general improvements to the code.