Adds STOP function.
Also changes 'break-now' behavior so that INVOKE-BREAK triggers
the end-of-break callback.
Revises Everything.

Adds more detail to the README; includes note about SDL_VIDEO_ALLOW_SCREENSAVER.

The main file is re-uses more general 'alert' callbacks.

Re-adds a revised set of StumpWM commands to the new stumpwm.lisp file.