... so that we can use Track::SharedPointer without undefined behavior even on
tracks that don't yet belong to any TrackList.
Also fix the return type of function template TrackList::Add and remove some
casts.
... grouping the channels; rather than one flat vector.
OGG, GStreamer and FFmpeg import were written to allow multiple multi-channel
tracks; others always imported one group of channels.
All of that is reflected in the results returned by the importers, though it
makes no difference yet in AudacityProject::AddImportedTracks (where the
results are used).