Obviate the need for explicit 'remember_tree_state/restore_tree_state' calls

Instead, just keep track of the expanded state of trips as we get the
gtk callbacks for the state changes (which we need to track anyway for
the selection logic), and automatically restore the state whenever we
re-create the divelist.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: Start calculations with gas 0 instead of air

The planner had a fatal flaw in that it ALWAYS started with AIR, even when
clearly claiming to use the correct gas. We only picked up correct gases
from events, but not at the beginning of the dive (where there is no
event).

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