Add support for visibility tracking and allow manual entry air temp

Turns out we had a data field for visibility as a length unit - but never
used it. I can never guess how much visibility we actually had on a dive -
but I think most everyone can assign a rating between abysmal (zero stars,
"I couldn't read my dive computer even right in front of my mask" - trust
me, I had some of those dives) to amazing ("five stars, I could see farther
than I though possible" - and I had one or two of those, too). So I
changed this to an integer and am re-using the star infrastructure we have
for the overall dive rating.

When displaying this I was dismayed that we are running out of space in
the "Dive Notes" notbook. So I moved this to the "Dive Info" notebook.
This is not consistent and not logical. I think we need to revisit the
notebooks and think about what we want to display where.

While adding the infrastructure to manually enter the visibility I went
ahead and added the ability to manually enter the air temperature as well
(that was one of the things missing in the previous commit).

Fixes #7

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

dive.h

@@ -267,7 +267,7 @@ struct dive {
 	double latitude, longitude;
 	depth_t maxdepth, meandepth;
 	duration_t duration, surfacetime;
-	depth_t visibility;
+	int visibility; /* 0 - 5 star rating */
 	temperature_t airtemp, watertemp;
 	cylinder_t cylinder[MAX_CYLINDERS];
 	weightsystem_t weightsystem[MAX_WEIGHTSYSTEMS];