Implement member functions of classes in corresponding .cpp files...

... in four cases; not in some other .cpp file.

This is another move that causes the generated graph to reflect dependencies
correctly.

This fixes other large, hidden cycles that involved PrefsDialog.cpp: there was
link dependency on that when PrefsPanel.h was used for the base class.  No
longer.

Also cycles involving TrackPanel.cpp, which contained the default
implementations for TrackPanelCell and related abstract base classes.

src/TrackPanelCell.cpp

@@ -0,0 +1,81 @@
+/**********************************************************************
+ 
+ Audacity: A Digital Audio Editor
+ 
+ TrackPanelCell.cpp
+ 
+ Paul Licameli split from TrackPanel.cpp
+ 
+ **********************************************************************/
+
+#include "TrackPanelCell.h"
+
+#include <wx/event.h>
+#include "HitTestResult.h"
+#include "RefreshCode.h"
+
+TrackPanelNode::TrackPanelNode()
+{
+}
+
+TrackPanelNode::~TrackPanelNode()
+{
+}
+
+TrackPanelGroup::TrackPanelGroup()
+{
+}
+
+TrackPanelGroup::~TrackPanelGroup()
+{
+}
+
+TrackPanelCell::~TrackPanelCell()
+{
+}
+
+HitTestPreview TrackPanelCell::DefaultPreview
+(const TrackPanelMouseState &, const AudacityProject *)
+{
+   return {};
+}
+
+unsigned TrackPanelCell::HandleWheelRotation
+(const TrackPanelMouseEvent &, AudacityProject *)
+{
+   return RefreshCode::Cancelled;
+}
+
+unsigned TrackPanelCell::DoContextMenu
+   (const wxRect &, wxWindow*, wxPoint *, AudacityProject*)
+{
+   return RefreshCode::RefreshNone;
+}
+
+unsigned TrackPanelCell::CaptureKey(
+   wxKeyEvent &event, ViewInfo &, wxWindow *, AudacityProject *)
+{
+   event.Skip();
+   return RefreshCode::RefreshNone;
+}
+
+unsigned TrackPanelCell::KeyDown(
+   wxKeyEvent &event, ViewInfo &, wxWindow *, AudacityProject *)
+{
+   event.Skip();
+   return RefreshCode::RefreshNone;
+}
+
+unsigned TrackPanelCell::KeyUp(
+   wxKeyEvent &event, ViewInfo &, wxWindow *, AudacityProject *)
+{
+   event.Skip();
+   return RefreshCode::RefreshNone;
+}
+
+unsigned TrackPanelCell::Char(
+   wxKeyEvent &event, ViewInfo &, wxWindow *, AudacityProject *)
+{
+   event.Skip();
+   return RefreshCode::RefreshNone;
+}