Show statistics of selected dives

Show statistics of selected dives

If at least 2 dives are selected, show statistics of these dives on
Overall Stats. Otherwise, show the statistics of all dives. Temperature
is also added to the shown statistics.

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Minor change to avoid adding statistics.h (moved the global variable and
external function declaration to display-gtk.h).
Another minor change to the text displayed for the "Stats" notebook page.

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