Move some handle and cell classes into their own files

src/TrackPanelResizerCell.cpp

@@ -0,0 +1,27 @@
+/**********************************************************************
+
+Audacity: A Digital Audio Editor
+
+TrackPanelResizeHandle.cpp
+
+Paul Licameli split from TrackPanel.cpp
+
+**********************************************************************/
+
+#include "Audacity.h"
+#include "TrackPanelResizerCell.h"
+#include "TrackPanelResizeHandle.h"
+#include "HitTestResult.h"
+
+TrackPanelResizerCell::TrackPanelResizerCell( std::shared_ptr<Track> pTrack )
+: mpTrack{ pTrack }
+{}
+
+HitTestResult TrackPanelResizerCell::HitTest
+(const TrackPanelMouseEvent &event, const AudacityProject *pProject)
+{
+   return {
+      TrackPanelResizeHandle::HitPreview( mBetweenTracks ),
+      &TrackPanelResizeHandle::Instance()
+   };
+}