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/ScreenEditCoursesMenu.cpp

@@ -16,7 +16,7 @@
 #define EXPLANATION_TEXT			THEME->GetMetric (m_sName,"ExplanationText")
 #define HELP_TEXT					THEME->GetMetric (m_sName,"HelpText")
 
-const AutoScreenMessage SM_RefreshSelector;
+AutoScreenMessage( SM_RefreshSelector )
 
 REGISTER_SCREEN_CLASS( ScreenEditCoursesMenu );
 ScreenEditCoursesMenu::ScreenEditCoursesMenu( CString sName ) : ScreenWithMenuElements( sName )