Dispatch read of top-level project XML tags with a table of functions...

... which makes Project.cpp a bit less dependent on some details of other
classes

This puts Tags.cpp back into the big strongly connected component of the
dependency graph.  That will be remedied later when Project.cpp becomes a
low-level file

src/TimeTrack.cpp

@@ -27,6 +27,7 @@
 #include "Envelope.h"
 #include "Prefs.h"
 #include "Project.h"
+#include "ProjectFileIORegistry.h"
 #include "TrackArtist.h"
 #include "AllThemeResources.h"
 
@@ -39,6 +40,15 @@ std::shared_ptr<TimeTrack> TrackFactory::NewTimeTrack()
    return std::make_shared<TimeTrack>(mDirManager, mZoomInfo);
 }
 
+static ProjectFileIORegistry::Entry registerFactory{
+   wxT( "timetrack" ),
+   []( AudacityProject &project ){
+      auto &trackFactory = *project.GetTrackFactory();
+      auto &tracks = *project.GetTracks();
+      return tracks.Add(trackFactory.NewTimeTrack());
+   }
+};
+
 TimeTrack::TimeTrack(const std::shared_ptr<DirManager> &projDirManager, const ZoomInfo *zoomInfo):
    Track(projDirManager)
    , mZoomInfo(zoomInfo)