Add Info & Stats page to the notebook

This provides the relevant information for the currently selected dive
plus a bunch of statistics over all dives in the dive_table.

The visual design has lots of room for improvement
- right now the different fields change size
- it might be nice to have a more modern look for the entries
- the O2/He field is odd - for most divers the He value will
  always be 0, so maybe we should only show He if there's at least one
  dive that uses He? Also, we simply do a comma separated list of gases
  for all the tanks used

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

display-gtk.h

@@ -33,6 +33,7 @@ extern GtkWidget *dive_profile_widget(void);
 extern GtkWidget *dive_info_frame(void);
 extern GtkWidget *extended_dive_info_widget(void);
 extern GtkWidget *equipment_widget(void);
+extern GtkWidget *stats_widget(void);
 
 extern GtkWidget *dive_list_create(void);