Make objects attached to AudacityProject non-copyable...

... except for Tags; also some uses of final and explicit

This caught a mistake in Scrubbing.cpp

src/tracks/ui/BackgroundCell.h

@@ -30,6 +30,7 @@ public:
    static BackgroundCell &Get( AudacityProject &project );
    static const BackgroundCell &Get( const AudacityProject &project );
 
+   explicit
    BackgroundCell(AudacityProject *pProject)
       : mpProject(pProject)
    {}

src/tracks/ui/EditCursorOverlay.h

@@ -22,6 +22,7 @@ class EditCursorOverlay final
    , public ClientData::Base
 {
 public:
+   explicit
    EditCursorOverlay(AudacityProject *project, bool isMaster = true);
 
 private:

src/tracks/ui/PlayIndicatorOverlay.h

@@ -50,6 +50,7 @@ protected:
 class PlayIndicatorOverlay final : public PlayIndicatorOverlayBase
 {
 public:
+   explicit
    PlayIndicatorOverlay(AudacityProject *project);
 
 private:

src/tracks/ui/Scrubbing.cpp

@@ -744,7 +744,7 @@ bool Scrubber::IsScrubbing() const
 {
    if (mScrubToken <= 0)
       return false;
-   auto projectAudioIO = ProjectAudioIO::Get( *mProject );
+   auto &projectAudioIO = ProjectAudioIO::Get( *mProject );
    if (mScrubToken == projectAudioIO.GetAudioIOToken() &&
             projectAudioIO.IsAudioActive())
       return true;

src/tracks/ui/Scrubbing.h

@@ -44,7 +44,10 @@ public:
    static Scrubber &Get( AudacityProject &project );
    static const Scrubber &Get( const AudacityProject &project );
 
+   explicit
    Scrubber(AudacityProject *project);
+   Scrubber( const Scrubber & ) PROHIBITED;
+   Scrubber &operator=( const Scrubber & ) PROHIBITED;
    ~Scrubber();
 
    static bool ShouldScrubPinned();
@@ -192,6 +195,7 @@ class ScrubbingOverlay final
    , public ClientData::Base
 {
 public:
+   explicit
    ScrubbingOverlay(AudacityProject *project);
 
 private:

src/tracks/ui/TrackView.cpp

@@ -166,7 +166,7 @@ namespace {
 
 // Attach an object to each project.  It receives track list events and updates
 // track Y coordinates
-struct TrackPositioner : ClientData::Base, wxEvtHandler
+struct TrackPositioner final : ClientData::Base, wxEvtHandler
 {
    AudacityProject &mProject;
 
@@ -182,6 +182,8 @@ struct TrackPositioner : ClientData::Base, wxEvtHandler
       TrackList::Get( project ).Bind(
          EVT_TRACKLIST_RESIZING, &TrackPositioner::OnUpdate, this );
    }
+   TrackPositioner( const TrackPositioner & ) PROHIBITED;
+   TrackPositioner &operator=( const TrackPositioner & ) PROHIBITED;
 
    void OnUpdate( TrackListEvent & e )
    {