Small changes to the selection logic

Now it correctly uses the existing helper functions and keeps our idea of
the selection consistent.

There is a small behavioral change compared to the Gtk code. Range
selections no longer have the last dive clicked on as selected_dive but
instead the dive with the highest index that was selected. I don't think
that is a major issue for anyone.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>