Move track resizing code out of TrackPanel.cpp, though it still...

... cooperates more closely with TrackPanel than the other UIHandle subclasses
do.

src/TrackPanelResizeHandle.h

@@ -0,0 +1,68 @@
+/**********************************************************************
+
+Audacity: A Digital Audio Editor
+
+TrackPanelResizeHandle.h
+
+Paul Licameli split from TrackPanel.cpp
+
+**********************************************************************/
+
+#ifndef __AUDACITY_TRACK_PANEL_RESIZE_HANDLE__
+#define __AUDACITY_TRACK_PANEL_RESIZE_HANDLE__
+
+#include "UIHandle.h"
+
+struct HitTestResult;
+class Track;
+
+class TrackPanelResizeHandle final : public UIHandle
+{
+   TrackPanelResizeHandle();
+   TrackPanelResizeHandle(const TrackPanelResizeHandle&) = delete;
+   TrackPanelResizeHandle &operator=(const TrackPanelResizeHandle&) = delete;
+
+public:
+   static TrackPanelResizeHandle& Instance();
+   static HitTestPreview HitPreview(bool bLinked);
+
+   virtual ~TrackPanelResizeHandle();
+
+   Result Click
+      (const TrackPanelMouseEvent &event, AudacityProject *pProject) override;
+
+   Result Drag
+      (const TrackPanelMouseEvent &event, AudacityProject *pProject) override;
+
+   HitTestPreview Preview
+      (const TrackPanelMouseEvent &event,  const AudacityProject *pProject)
+      override;
+
+   Result Release
+      (const TrackPanelMouseEvent &event, AudacityProject *pProject,
+       wxWindow *pParent) override;
+
+   Result Cancel(AudacityProject *pProject) override;
+
+   void OnProjectChange(AudacityProject *pProject) override;
+
+private:
+   enum Mode {
+      IsResizing,
+      IsResizingBetweenLinkedTracks,
+      IsResizingBelowLinkedTracks,
+   };
+   Mode mMode{ IsResizing };
+
+   Track *mpTrack{};
+
+   bool mInitialMinimized{};
+   int mInitialTrackHeight{};
+   int mInitialActualHeight{};
+   int mInitialUpperTrackHeight{};
+   int mInitialUpperActualHeight{};
+
+   int mMouseClickY{};
+};
+
+#endif