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/ScreenEz2SelectMusic.cpp

@@ -64,7 +64,7 @@
 
 const float TWEEN_TIME		= 0.5f;
 
-const AutoScreenMessage SM_NoSongs;
+AutoScreenMessage( SM_NoSongs )
 
 REGISTER_SCREEN_CLASS( ScreenEz2SelectMusic );
 ScreenEz2SelectMusic::ScreenEz2SelectMusic( CString sName ) : ScreenWithMenuElements( sName )