Allow sorting dives by date - without the dive trips in the dive list

[Linus Torvalds]

We used to enable dive trips whenever we sorted by date, which can be
a bit annoying. Sometimes you really just want to sort all your dives
by date, without necessarily seeing the trip data.

So this changes the default sort to be the "dive number" table, and
then does *not* actually sort by the dive number, but instead enables
the trips, and then sorts the result by date. So the "dive number"
column - which used to be non-sortable - becomes semantically
equivalent to the old date column sorting.

And now sorting by date makes it act like sorting by depth or any
other attribute - we hide the dive trip tree, and just show the plain
list of dives (sorted by date, obviously).

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

This is not all that heavily tested, but does what I expected/wanted
for the small testing I did.
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