Correctly implement Metric / Imperial / Personalize preference

The code so far had completely ignored Metric / Imperial. Turning this
into a three way radio box seemed to make much more sense.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Once again try to fix the selection

Things got broken. Again. We no longer kept track of the selected dives in
our structures which broke statistics.

This attempts to fix that, but appears to still have a bug when selecting
trips. Sometimes this results in 0 dives being selected according to our
data structures, while Qt happily shows all dives of the trip as seected.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Merge branch 'selection'