We should never show decimals on depth in feet

That's 3cm resolution - just unrealistic

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Divelist: switch to better column headers

And move the units there.
This is a long going back and forth. What we want is narrow columns. But
what we need is something that's easy to understand for our users.

I'm open to other suggestions, but I think this moves us in the right
direction.

See #712

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Divelist: Use more sensible default column widths

Having 100px wide columns was simplistic and stupid. It was never intended
to stay that way.

See #712

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Use the Windows default font on Windows 7 and 8/8.1

It contains the characters we need and will make the application look more
"native".

See #712

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