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/ScreenNetEvaluation.cpp

@@ -17,7 +17,7 @@ const int NUM_SCORE_DIGITS	=	9;
 
 #define MAX_COMBO_NUM_DIGITS		THEME->GetMetricI("ScreenEvaluation","MaxComboNumDigits")
 
-const AutoScreenMessage SM_GotEval;
+AutoScreenMessage( SM_GotEval ) 
 
 REGISTER_SCREEN_CLASS( ScreenNetEvaluation );
 ScreenNetEvaluation::ScreenNetEvaluation (const CString & sClassName) : ScreenEvaluation( sClassName )