Add signal handlers

Add signal handlers

This adds a signal handler which calls gtk_main_quit() for the following
signals: SIGINT, SIGQUIT, SIGABRT, SIGTERM, SIGKILL. It gives us a place
to clean up, even if we have a problem.

If handling is needed for more signals, they can be added later. They can
be differentiated in the signal handler by using the signum variable.