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/TrackArtist.h

@@ -37,7 +37,7 @@ class LabelTrack;
 class TimeTrack;
 class TrackList;
 class Ruler;
-struct ViewInfo;
+class ViewInfo;
 
 #ifndef uchar
 typedef unsigned char uchar;