static ::Get functions for overlays on TrackPanel...

... not member functions of AudacityProject.

AdornedRulerPanel is not (yet) made similarly independent of its overlay
code.

Two .cpp files implementing overlays escape dependency cycles to higher levels.

src/Project.h

@@ -55,7 +55,6 @@ class AudacityProject;
 class AutoSaveFile;
 class Importer;
 class ODLock;
-class Overlay;
 class RecordingRecoveryHandler;
 namespace ProjectFileIORegistry{ struct Entry; }
 class TrackList;
@@ -649,15 +648,6 @@ public:
 private:
    bool mbInitializingScrollbar{ false };
 
-   // TrackPanelOverlay objects
-   std::shared_ptr<Overlay>
-      mIndicatorOverlay, mCursorOverlay;
-
-#ifdef EXPERIMENTAL_SCRUBBING_BASIC
-   std::shared_ptr<Overlay> mScrubOverlay;
-private:
-#endif
-
 public:
    class PlaybackScroller final : public wxEvtHandler
    {