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/ScreenTestFonts.cpp

@@ -17,7 +17,8 @@ void ChangeText(CString txt)
 {
 	CustomText = txt;
 }
-const AutoScreenMessage SM_ChangeText;
+
+AutoScreenMessage( SM_ChangeText )
 
 void ScreenTestFonts::HandleScreenMessage( const ScreenMessage SM )
 {