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

[Dirk Hohndel]

Linus Torvalds <torvalds at linux-foundation.org> writes:

> 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).

Looks sane. Tested it with a few files and random switches of sort
order.

Pushed.

/D