Improve divelist group header information

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>