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/ScreenSelectMaster.cpp

@@ -14,8 +14,7 @@
 #include "Foreach.h"
 #include "RageSoundManager.h"
 
-
-const AutoScreenMessage SM_PlayPostSwitchPage;
+AutoScreenMessage( SM_PlayPostSwitchPage )
 
 CString CURSOR_OFFSET_X_FROM_ICON_NAME( size_t p, size_t part ) { return ssprintf("CursorPart%dP%dOffsetXFromIcon",int(part+1),int(p+1)); }
 CString CURSOR_OFFSET_Y_FROM_ICON_NAME( size_t p, size_t part ) { return ssprintf("CursorPart%dP%dOffsetYFromIcon",int(part+1),int(p+1)); }