Add typical 0 to 5 star rating for dives

This works ok-ish, but doesn't allow us to click on the stars and edit
them in the divelist, which a user might expect to be able to do - in
most "star rating UIs" you simply click on the n-th star to set that
rating. Here you need to edit the dive and pick the rating from a drop
down menu.

Minor oddity: you can actually (if you force it) write anything you want
into the star rating. But anything that isn't one of the predefined
strings simply results in a zero star rating.

Overall the UI feels a bit... forced. But I think this is quite useful
anyway.

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

save-xml.c

@@ -182,6 +182,7 @@ static void save_overview(FILE *f, struct dive *dive)
 	show_location(f, dive);
 	show_utf8(f, dive->divemaster, "  <divemaster>","</divemaster>\n");
 	show_utf8(f, dive->buddy, "  <buddy>","</buddy>\n");
+	fprintf(f, "  <rating>%d</rating>\n", dive->rating);
 	show_utf8(f, dive->notes, "  <notes>","</notes>\n");
 }