GetProjectPanel analogous to GetProjectFrame breaks dependencies...

... in places that need the TrackPanel but only to invoke common wxWindow
methods on it.

This eliminates direct use of TrackPanel by Scrubbing and ProjectWindow

src/TrackPanel.cpp

@@ -205,7 +205,7 @@ AudacityProject::AttachedWindows::RegisteredFactory sKey{
       wxASSERT( mainPage ); // to justify safenew
 
       auto &tracks = TrackList::Get( project );
-      return safenew TrackPanel(mainPage,
+      auto result = safenew TrackPanel(mainPage,
          window.NextWindowID(),
          wxDefaultPosition,
          wxDefaultSize,
@@ -213,6 +213,8 @@ AudacityProject::AttachedWindows::RegisteredFactory sKey{
          &viewInfo,
          &project,
          &ruler);
+      project.SetPanel( result );
+      return result;
    }
 };