Refactor and run-time control of keyclick / bell

- Major refactor for both readability and maintainability.

- Keyclick and Bell can be toggled from the keyboard by pressing
  SETUP + K to enable/disable keyclick, and SETUP + B to
  enable/disable bell.

- Since SETUP was being used for OS X "Command" key, decided
  to sacrifice both PF3 and PF4 for "Alt" and "Command",
  respectively.
Untabifying
Moved defines to header file