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/ProjectManager.cpp

@@ -1081,3 +1081,14 @@ int ProjectManager::GetEstimatedRecordingMinsLeftOnDisk(long lCaptureChannels) {
    int iRecMins = (int)round(dRecTime / 60.0);
    return iRecMins;
 }
+
+/// This was moved here to eliminate dependency of Scrubbing.cpp on
+/// TrackPanel, but perhaps a better home should be found for it in future
+#include "tracks/ui/Scrubbing.h"
+static const AudacityProject::AttachedObjects::RegisteredFactory sOverlayKey{
+  []( AudacityProject &parent ){
+     auto result = std::make_shared< ScrubbingOverlay >( &parent );
+     TrackPanel::Get( parent ).AddOverlay( result );
+     return result;
+   }
+};