instead of loading actors inside OptionRow, load a single copy of each

one we need, and then make copies as needed.  This avoids lots of
extra loading (which can be slow, even with refcounted data; lots of path
lookups, etc).  (Trying to get this to the point where we can construct new
OptionRows for a menu without making a whole new menu.)  This approach
means we don't lose the potential to have different sTypes of OptionRow
in a single menu.  (The annoying bit is that any actor we copy has to handle
copying, which is tricky for ActorFrame and doesn't work yet for AutoActor.
Working on it ...)

stepmania/src/ScreenOptions.h

@@ -36,6 +36,8 @@ public:
 	virtual void HandleScreenMessage( const ScreenMessage SM );
 
 protected:
+	OptionRowType m_OptionRowType;
+
 	virtual void TweenOnScreen();
 
 	virtual void ImportOptions( int iRow, const vector<PlayerNumber> &vpns ) = 0;