Apply sort functions to the correct model, don't select summary entries

We only set up the column specific sort functions for the default (tree)
model, which caused us to not sort correctly in the list model.

This commit also somewhat cleans up the handling of selecting summary
lines in the tree model, which includes the very first selection made at
program start (which happens to be the very last dive).

But it still doesn't work the way I expect it to work (i.e., the correct
row is not highlighted). Fundamentally I would prefer clicks on the
summary lines to instead select (or as ctrl-click, possibly deselect) all
the dives under that summary entry. Still TODO.

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