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/ScreenEvaluation.cpp

@@ -83,8 +83,8 @@ const char* STATS_STRING[NUM_STATS_LINES] =
 
 static const int NUM_SHOWN_RADAR_CATEGORIES = 5;
 
-const AutoScreenMessage SM_PlayCheer;
-const AutoScreenMessage SM_AddBonus;
+AutoScreenMessage( SM_PlayCheer )
+AutoScreenMessage( SM_AddBonus )
 
 
 REGISTER_SCREEN_CLASS( ScreenEvaluation );