Bug834, part 2 - Memory leak and orphans after cancelling any generator

New project.  New track.  Generate sound -- do not cancel.  Select all,
generate sound again -- this time, do cancel the progress dialog.

Undo, so you see one, empty track again.  Save.  Close.  Reopen.

The cause of these orphans is different from the first case.  The previous
commit is necessary but not sufficient to fix this.

src/Track.cpp

@@ -700,8 +700,9 @@ Track *SyncLockedTracksIterator::Last(bool skiplinked)
 DEFINE_EVENT_TYPE(EVT_TRACKLIST_RESIZED);
 DEFINE_EVENT_TYPE(EVT_TRACKLIST_UPDATED);
 
-TrackList::TrackList()
+TrackList::TrackList(bool destructorDeletesTracks)
 :  wxEvtHandler()
+, mDestructorDeletesTracks(destructorDeletesTracks)
 {
    head = NULL;
    tail = NULL;
@@ -709,7 +710,7 @@ TrackList::TrackList()
 
 TrackList::~TrackList()
 {
-   Clear();
+   Clear(mDestructorDeletesTracks);
 }
 
 void TrackList::RecalcPositions(const TrackListNode *node)