Clean up some typos

Cosmetic commit to clean up some of the annoying typos in qt-ui

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix inaccurate weight and temperature display in dive list

A nonexisting temperature (mkelvin==0) was displayed as -273°C.
Weight was always displayed with an extra 500 grams/0.5 lbs.

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>