TrackPanel simplifications; improved cursor handling...

... in cases such as:  dragging an envelope point, then hitting space to play,
which forces the drag to finish early.  If you move the mouse, cursor will
remain "ban", even while the mouse button remains down but no drag is really
happening.

src/TrackPanelMouseEvent.h

@@ -22,12 +22,12 @@ struct TrackPanelMouseEvent
 {
    TrackPanelMouseEvent
       ( wxMouseEvent &event_, const wxRect &rect_, const wxSize &whole_,
-        TrackPanelCell *pCell_ = NULL, double steps_ = 0.0 )
+        TrackPanelCell *pCell_ )
       : event{ event_ }
       , rect{ rect_ }
       , whole{ whole_ }
       , pCell{ pCell_ }
-      , steps{ steps_ }
+      , steps{ 0 }
    {
    }