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>

dive.c

@@ -559,6 +559,7 @@ struct dive *try_to_merge(struct dive *a, struct dive *b)
 	MERGE_TXT(res, a, b, notes);
 	MERGE_TXT(res, a, b, buddy);
 	MERGE_TXT(res, a, b, divemaster);
+	MERGE_MAX(res, a, b, rating);
 	MERGE_MAX(res, a, b, number);
 	MERGE_MAX(res, a, b, maxdepth.mm);
 	res->meandepth.mm = 0;