This avoids constructing xml by hand in both the controller and
the model, and opens the way for other rendering in future.
The complexity of deciding which point goes where, along with revisiting
previous tracks and tracksegs means that I've broken it down into
two parts - sorting the points into the right trksegs is done first,
before rendering them all as xml. I couldn't find a way to allow
revisiting using the builder.
95e5178bfb