Explanation of signals 

When something happens to the application 
	the mouse is moved
	a button was clicked by the mouse
	some key was pressed (or released) on the keyboard
	the application was closed via the [x] on the window
our application receives a "signal" appropriate to the event.
If we have the appropriate "signal handlers" in place when such
signal is received then our application (the Gtk2->main loop)
calls that "signal handler".

Within the signal handler we can do any processing.
Once the signal handler is finished the main loop gets back the command
and waits for the next signal to arrive. (That is, unless we exit the
program from within that signal handler.

Now we can go back to the first example and rememver we had this magic
line in it:

    $window->signal_connect (destroy => sub { Gtk2->main_quit; });

This is actually a signal handler that will be called when the window is destroyed,
that is when you click on the [x] on the window.
Within the signal handler you can see an anonymous subroutin with the single command 
in it to quite the Gtk2 application.	
Without this line when you click on [x] the window will disappear but the application
will not quit. If you executed it from the command line you can kill it by
Ctr-C but it will seem it "hangs".

Throughout the tutorial we will see many types of signals and many ways to handle those
signals.