Define TrackPanelOverlay, use it to reimplement play indicator and cursor...

... but scrub speed display is disabled

src/TrackPanelCell.h

@@ -0,0 +1,19 @@
+/**********************************************************************
+
+Audacity: A Digital Audio Editor
+
+TrackPanelCell.h
+
+Paul Licameli
+
+**********************************************************************/
+
+#ifndef __AUDACITY_TRACK_PANEL_CELL__
+#define __AUDACITY_TRACK_PANEL_CELL__
+
+// Future:  TrackPanelCell will be generalized to a new abstract base class of Track
+// and of other things.
+class Track;
+using TrackPanelCell = Track;
+
+#endif