Improve tree model implementation

We now support three hierarchy levels: day, month, and year. Each
indicated by a negative DIVE_INDEX for -1 to -3. This allows a nice
compact overview when doing date based sorting (the default).

As indicated in the previous commit, things still go wrong with sorting by
other columns as the entries are only sorted within each day, not globally
across the whole dive list.

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