Modify AutoScreenMessage to keep map of all non-standard ScreenMessages.

This has a lot of advantages over the old ways.  Some of the initial ones are that
you can log a list of all screenmessages and their respective number and another
is that it would be fairly easy to add a reverse lookup, so you could get the name
of a ScreenMessage for log use by getting the ScreenMessage number.

stepmania/src/ScreenDemonstration.cpp

@@ -14,7 +14,6 @@
 #define SECONDS_TO_SHOW			THEME->GetMetricF("ScreenDemonstration","SecondsToShow")
 #define NEXT_SCREEN				THEME->GetMetric("ScreenDemonstration","NextScreen")
 
-
 REGISTER_SCREEN_CLASS( ScreenDemonstration );
 ScreenDemonstration::ScreenDemonstration( CString sName ) : ScreenJukebox( sName )
 {