Don't add spurious 5m/15ft node at 10 minutes when canceling edit

When cancelling the edit of a manually added dive, we should send the
sample with timestamp 0 to addStop - that's a magic value and causes us to
add a node at 5m/15ft and 10 minutes instead. Since the dive always starts
at the surface, calling addStop is redundant, anyway, so let's just not do
that.

See #270

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix various issues with the dive add / edit manual dive code

- get_gas_from_events does NOT always set o2/he. It only updates them IFF
  a matching event is found; so we need to make sure we start out with a
  valid gas mix
- the way we tried to restore the edited dive in case of an edit to a
  manually added that is cancelled was completely bogus. Way too complex
  when we can simply and reliably simply store the dive and then copy it
  back

Fixes #270

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