Define TrackShifter, subclasses, factory method; collect them...

... in TimeShiftHandle.  They do nothing useful yet, but responsibilities
will shift into them.

TypeSwitch is avoided and the subclasses are defined in appropriate places
for dependency management, thanks to AttachedVirtualFunction.

src/tracks/playabletrack/wavetrack/ui/WaveTrackView.cpp

@@ -1301,3 +1301,24 @@ void WaveTrackView::Draw(
 
    CommonTrackView::Draw( context, rect, iPass );
 }
+
+class WaveTrackShifter final : public TrackShifter {
+public:
+   WaveTrackShifter( WaveTrack &track )
+      : mpTrack{ track.SharedPointer<WaveTrack>() }
+   {
+   }
+   ~WaveTrackShifter() override {}
+   Track &GetTrack() const override { return *mpTrack; }
+
+private:
+   std::shared_ptr<WaveTrack> mpTrack;
+};
+
+using MakeWaveTrackShifter = MakeTrackShifter::Override<WaveTrack>;
+template<> template<> auto MakeWaveTrackShifter::Implementation() -> Function {
+   return [](WaveTrack &track) {
+      return std::make_unique<WaveTrackShifter>(track);
+   };
+}
+static MakeWaveTrackShifter registerMakeWaveTrackShifter;