Simplify and clean up dive trip management

[Linus Torvalds]

This adds a couple of helper functions to manage dive trips
("add_dive_to_trip()" and "remove_dive_from_trip()") and makes those
functions do the trip statistics maintenance (trip beginning times,
number of dives, etc).

This was needed because the dive merge cases for multiple dive
computers showed some rather nasty special cases: especially if the
new dive information has been loaded into an XML file with trips
auto-generated, merging several of these kinds of xml files with
multiple dives in several overlapping trips would completely confuse
our previous code.

In particular, auto-generated trips that had the exact same date as
previous trips (because they were generated from the same dive
computer) really confused the code that used the trip timestamp to
manage the trips.

Adding the helper functions allows us to get the general case right
without having to have each piece of code that handles trip
information having to bother about all the odd rules. It will
eventually also allow us to make the dive trip data structures more
logical: right now the dive trip list is largely designed around the
odd gtk model handling, rather than some more higher-level conceptual
relationship with the actual dives.

But for now, this keeps all the data structures unchanged, and just
modifies them using the new helper functions.

Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
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