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/ScreenRanking.cpp

@@ -43,8 +43,8 @@ XToString( PageType, NUM_PAGE_TYPES );
 #define COURSE_DIFFICULTY_X(col)	(COURSE_DIFFICULTY_START_X+COL_SPACING_X*col)
 #define COURSE_SCORE_OFFSET_X(col)	(COURSE_SCORE_OFFSET_START_X+COL_SPACING_X*col)
 
-const AutoScreenMessage SM_ShowNextPage;
-const AutoScreenMessage SM_HidePage;
+AutoScreenMessage( SM_ShowNextPage )
+AutoScreenMessage( SM_HidePage )
 
 
 static void GetAllSongsToShow( vector<Song*> &vpOut )