Refactor prepared screens. This can handle:

- preparing and reusing screens used in a loop (eg. attract), a conceptual stack
   (eg. options menu), or a real stack (ScreenPrompt)
 - automatic cleanup once you load a screen that isn't preloaded; this gracefully
   handles unusual or forgotten exit paths (eg. pressing the system menu button)
 - reusing some screens in a group and loading others on demand

stepmania/src/ScreenManager.h

@@ -32,6 +32,7 @@ public:
 	void SetNewScreen( const CString &sName );
 	void AddNewScreenToTop( const CString &sName, ScreenMessage SendOnPop=SM_None );
 	void PrepareScreen( const CString &sScreenName );	// creates and caches screen so that the next call to SetNewScreen for the prep'd screen will be very quick.
+	void GroupScreen( const CString &sScreenName );
 	bool ConcurrentlyPrepareScreen( const CString &sScreenName, ScreenMessage send_when_done = SM_None );
 	bool IsConcurrentlyLoading() const;
 	void DeletePreparedScreens();
@@ -65,6 +66,8 @@ public:
 	void	PlaySharedBackgroundOffCommand();
 	void    ZeroNextUpdate();
 private:
+	void AboutToLoadScreen( const CString &sScreenName );
+
 	Screen				*m_pInputFocus; // NULL = top of m_ScreenStack
 
 	CString				m_sSystemMessage;
@@ -79,8 +82,9 @@ private:
 	// operations take a long time, and will cause a skip on the next update.
 	bool				m_bZeroNextUpdate;
 
-	void ClearScreenStack();
 	void LoadDelayedScreen();
+	void PrepareScreenInternal( const CString &sScreenName );
+	ScreenMessage PopTopScreenInternal();
 
 	// Keep these sounds always loaded, because they could be 
 	// played at any time.  We want to eliminate SOUND->PlayOnce