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/ScreenSplash.cpp

@@ -3,7 +3,7 @@
 #include "ThemeManager.h"
 #include "RageUtil.h"
 
-const AutoScreenMessage	SM_PrepScreen;
+AutoScreenMessage( SM_PrepScreen )
 
 
 REGISTER_SCREEN_CLASS( ScreenSplash );