Simplistic first attempt to get changes saved when quitting subsurface

Track whether things changed in the global dive_list

So far this actually works if changing dive info (but only if dive
selected was changed after the dive info was changed).

We are not tracking changes to the cylinder information, yet.

also remove the duplicate static dive_list

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