ViewInfo is becoming a smart class, not a dumb struct

Now it has:
A constructor
XML attribute serializer functions (but no XML tag of its own)

Also removed unused lastZoom

src/widgets/AttachableScrollBar.h

@@ -19,7 +19,7 @@
 
 #include <wx/scrolbar.h>
 
-struct ViewInfo;
+class ViewInfo;
 
 class AUDACITY_DLL_API AttachableScrollBar :
    public wxScrollBar

src/widgets/Ruler.h

@@ -19,7 +19,7 @@
 #include "../Envelope.h"
 #include "../Experimental.h"
 
-struct ViewInfo;
+class ViewInfo;
 class AudacityProject;
 class TimeTrack;
 class SnapManager;