Maintain selected rows when switching between list model and tree model

We keep track of the DIVE_INDEX of all selected dives and simply re-select
those dives after changing model (date based sort or sort by other
column).

There are a few TODOs left. We lose the sort direction (ascending /
descending) when switching models. We also don't correctly deal with the
user selecting summary rows in the tree model.

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