Separate out single dive statistics and total statistics

Right now this just changes the infrastructure - nothing outside of
statistics.c is modified. This is simply in preparation to split out the
single dive info and the total dive statistics in the future (as we are
creating more info and more stats and they will overflow the screen area
available - so this will turn into two notebook tabs).

This commit does not change any functionality.

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