Add Callbacks and Menus

I've combined what I believe to be the best parts from https://github.com/maccman/macgap/pull/14 and https://github.com/maccman/macgap/pull/80 (thanks to those respective authors) and modified it all to work with MacGap HEAD, thus providing comprehensive Main Menu control from Javascript.

This pull request contains ONLY menu-related code (and the underlying callback code which will likely prove useful in the future).