Use gas_mod and fix fpe

When removing o2 from a gas, eg setting it to "", air, this code would
raise a nice fpe. Fix that by using gas_mod instead.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Rework slot to quiet down Qt5 warning

Qt5 got confused about this slot. This does the same as previously but
gets Qt5 happy.

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>