Move responsibilities from Track to TrackView classes...

... And Track no longer inherits TrackPanelCell, so be careful to rewrite
some dynamic_casts too to check instead for TrackView.  Those casts won't fail
to recompile if not rewritten.

src/Track.cpp

@@ -387,11 +387,6 @@ void Track::SyncLockAdjust(double oldT1, double newT1)
    }
 }
 
-std::shared_ptr<Track> Track::DoFindTrack()
-{
-   return SharedPointer();
-}
-
 void PlayableTrack::Init( const PlayableTrack &orig )
 {
    mMute = orig.mMute;