Select better (?) default date for adding new dive

We now pick one hour after the end of the currently selected dive as the
default starting time for the new dive to be added.  If multiple dives
(or no dives) are selected, we default to current time as before.

The "one hour after the end" is just a random (but not unreasonable)
assumption for the surface time if you add multiple dives.

Suggested-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>