More things in this commit:

- Removed the callback system in favor of simply defining functions in a header to later be implemented in whichever file is most convenient for what we want to do (i.e., calling LLWindow callbacks within LLWindowMacOSX, setting cursors in llwindowmacosx-objc.mm, etc.)
- Viewer shutdown now works appropriately
- Added a bit of debugging code to test if a key has been handled by the UI or not (useful for tracking down the mystery of the enter key not being handled)
- Setup a cocoa quit handler within the application delegate that intercepts any termination requests