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/ScreenPlayerOptions.h

@@ -3,7 +3,7 @@
 
 #include "ScreenOptionsMaster.h"
 
-extern const AutoScreenMessage SM_BackFromPlayerOptions;
+AutoScreenMessage( SM_BackFromPlayerOptions )
 
 class ScreenPlayerOptions : public ScreenOptionsMaster
 {