TrackSplitter now handles properly fusions followed by gaps.

Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
TrackSplitter also builds the list of links it cuts.

So that you can use it to rebuild the track from the branches and the
links.
While at it: handle an extra special case where complex points also
involve gaps.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>
Polish TrackSplitter.

Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>