Use Subsurface's data structures for DiveItem

We have these data structures for a reason. They provide context
about the units used and prevent mistakes. And of course they
are used everywhere else so we should use them here, too.

This also tries to display some more data and make things look
a bit more like the Gtk version when it comes to alignment and
formatting.

My guess is this will make Qt developers' eyes bleed. My apologies.

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