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/ScreenStage.cpp

@@ -19,7 +19,7 @@
 #define PREV_SCREEN				THEME->GetMetric (m_sName,"PrevScreen")
 #define MINIMUM_DELAY			THEME->GetMetricF(m_sName,"MinimumDelay")
 
-const AutoScreenMessage	SM_PrepScreen;
+AutoScreenMessage( SM_PrepScreen )
 
 REGISTER_SCREEN_CLASS( ScreenStage );
 ScreenStage::ScreenStage( CString sClassName ) : Screen( sClassName )