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/EditCoursesMenu.cpp

@@ -34,7 +34,7 @@
 #define ROW_VALUE_X( i )		THEME->GetMetricF("EditCoursesMenu",ssprintf("RowValue%dX",i+1))
 #define ROW_Y( i )				THEME->GetMetricF("EditCoursesMenu",ssprintf("Row%dY",i+1))
 
-const AutoScreenMessage SM_BackFromCourseOptionsMenu;
+AutoScreenMessage( SM_BackFromCourseOptionsMenu )
 							  
 enum CourseEntryMenuRow
 {