Change default behavior for Stats to show selected dives

Previously when only one dive was selected, the Stats notebook page would show
the statistics for all dive. That creates a very illogical behavior when
clicking on the different dive groups in the dive list. The stats page would
always show how many dives where in a group when the group was selected, except
when there was only one dive in the group, in which case the statistics for all
the dives were shown.

With this change we also show the statistics for the selected dives, even if it
is just one. If you want the statistics for all dives, simply select them all
(Ctrl-A or Command-A on a Mac).

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>