To make sorting more controlled, move all sorting functions into
the core. For this, introduce a "dive_or_trip" structure, which
represents a top-level item. Adapt the DiveTripModel accordingly.
There are now three sorting functions:
1) dive_less_than
2) trip_less_than
3) dive_or_trip_less_than
These should be used by all sorting code. By moving them to a
single place, the mess can hopefully be cleaned up.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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