Identify below floor event type properly.

This event is on when accumulating deco time. Once you reach the floor
deco time will start decreasing and the event will stop. Going below the
floor again will re-activate the event.

Also identify event type 13 in DM4 imports as airtime.

Signed-off-by: Michael Andreen <harv@ruin.nu>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>