Allow date based grouping

This is the very first rough cut. It switches things over to a tree model
so we can have date based summary nodes.

It uses a DIVE_INDEX of -1 for summary nodes to easily tell them apart
from actual dives. All the data functions are changed so the summary
nodes only show the date they cover.

The commit also adds a couple of debug functions to be able to easily peek
into the model from the debugger.

Lots of things left to do. There is no longer a first dive selected when
starting subsurface. Sorting by columns other than date is messed up. We
almost certainly want month and year summary entries as well.

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