Improve divelist group header information

This shows the number of dives in the grup in the divelist header field,
and also picks the location from the first dive that *had* a location,
so that if any dive in the group has a valid location, the group will
have a location.

It also makes double-clicking a dive group expand/collapse that group.

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