More use of shared_ptr to Track, for safety...

... This even makes it possible to remove the prohibition of undo and redo
during transport, not that we want to though.  Playback and recording will
continue, using track objects that might not be in the current project.

src/Mix.h

@@ -23,6 +23,7 @@
 #include "MemoryX.h"
 #include <wx/string.h>
 #include "SampleFormat.h"
+#include <vector>
 
 class Resample;
 class DirManager;
@@ -30,7 +31,7 @@ class TimeTrack;
 class TrackFactory;
 class TrackList;
 class WaveTrack;
-class WaveTrackConstArray;
+using WaveTrackConstArray = std::vector < std::shared_ptr < const WaveTrack > >;
 class WaveTrackCache;
 
 /** @brief Mixes together all input tracks, applying any envelopes, amplitude