Separate Gtk related code from core logic: divelist

This is simplistic & brute force: any function that touches Gtk related
data structures is moved to divelist-gtk.c, everything else stays in
divelist.c.

Header files have been adjusted so that this still compiles and appears to
work. More thought is needed to truly abstract this out, but this seems to
be a good point to commit this change.

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

dive.h

@@ -410,7 +410,7 @@ static inline int rel_mbar_to_depth(int mbar, struct dive *dive)
  * be able to edit a dive without unintended side effects */
 extern struct dive edit_dive;
 
-extern gboolean autogroup;
+extern short autogroup;
 /* random threashold: three days without diving -> new trip
  * this works very well for people who usually dive as part of a trip and don't
  * regularly dive at a local facility; this is why trips are an optional feature */