Planner: Add backgas breaks

If the corresponding checkbox is checked the planner does interrupt
pure O2 deco after 12min for 6min on cylinder 0.

To make this work for air I removed the gasmix_is_null logic.

I guess that makes the planner feature complete for the next release.

[Dirk Hohndel: trivial merge into latest master]

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
UI restructure: add back the missing plan dive notes

This was removed in the big rewrite by mistake.

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