Figure out what is our first selected element (in case we start out from a
multiple selection) and then move to the next logical element. So the code
traverses an expanded tree (from a trip 'down' to its first dive or 'up'
to the last dive of the previous trip - and similar from a first dive in a
trip 'up' to its trip and from a last dive in a trip 'down' to the next
trip.
This does not take 'shift-cursor-up/down' into account (i.e. manual
selection extension). Instead with just cursor up and down a single dive
(or single trip) is selected.
My guess is that the code will make someone's eyes bleed. Be warned.
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
c729ae92ed