Move some handle and cell classes into their own files

src/TrackPanelResizerCell.h

@@ -0,0 +1,36 @@
+/**********************************************************************
+
+ Audacity: A Digital Audio Editor
+
+ TrackPanelResizerCell.h
+
+ Paul Licameli split from TrackPanel.cpp
+
+ **********************************************************************/
+
+#ifndef __AUDACITY_TRACK_PANEL_RESIZER_CELL__
+#define __AUDACITY_TRACK_PANEL_RESIZER_CELL__
+
+#include "tracks/ui/CommonTrackPanelCell.h"
+
+class TrackPanelResizerCell : public CommonTrackPanelCell
+{
+   TrackPanelResizerCell(const TrackPanelResizerCell&) = delete;
+   TrackPanelResizerCell &operator= (const TrackPanelResizerCell&) = delete;
+public:
+
+   explicit
+   TrackPanelResizerCell( std::shared_ptr<Track> pTrack );
+
+   HitTestResult HitTest
+   (const TrackPanelMouseEvent &event,
+    const AudacityProject *pProject) override;
+
+   std::shared_ptr<Track> FindTrack() override { return mpTrack.lock(); };
+private:
+   friend class TrackPanelCellIterator;
+   std::weak_ptr<Track> mpTrack;
+   bool mBetweenTracks {};
+};
+
+#endif