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/ScreenNameEntryTraditional.cpp

@@ -36,7 +36,7 @@ static const ThemeMetric<apActorCommands>	OK_INIT_COMMMAND		("ScreenNameEntryTra
 	if( !actor.GetName().empty() ) \
 		COMMAND( actor, command_name );
 
-const AutoScreenMessage	SM_ChangeDisplayedFeat;
+AutoScreenMessage( SM_ChangeDisplayedFeat )
 
 static const int CHAR_OK = -1;
 static const int CHAR_BACK = -2;