Bug1651: NoteTrack sync-lock and crash fixes...

Simplify the logic of duplication of NoteTrack.  Duplicates are always in
serialized state.  Un-serialization can happen on demand in any of the
NoteTrack operations that require a defined sequence.

Changing the duration of the sequence after paste is needed,
as it was also needed, when I fixed Stretch at commit
90eb4ec142.  I don't know if this should be
considered a bug in Allegro that we are compensating.

src/import/ImportMIDI.cpp

@@ -62,7 +62,7 @@ bool ImportMIDI(const wxString &fName, NoteTrack * dest)
    dest->SetName(trackNameBase);
    mf.Close();
    // the mean pitch should be somewhere in the middle of the display
-   Alg_iterator iterator(dest->GetSequence(), false);
+   Alg_iterator iterator( &dest->GetSeq(), false );
    iterator.begin();
    // for every event
    Alg_event_ptr evt;