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/ScreenStyleSplash.cpp

@@ -16,8 +16,8 @@
 #define NONSTOP_SCREEN				THEME->GetMetric("ScreenStyleSplash","NonstopScreen")
 #define ONI_SCREEN				THEME->GetMetric("ScreenStyleSplash","OniScreen")
 
-const AutoScreenMessage SM_DoneOpening;
-const AutoScreenMessage SM_StartClosing;
+AutoScreenMessage( SM_DoneOpening )
+AutoScreenMessage( SM_StartClosing )
 
 
 REGISTER_SCREEN_CLASS( ScreenStyleSplash );