* Nasal/mk-ap-helper.nas: prevent feedback loops between listeners for the
cockpit switches propagating property changes from /autopilot/switches/* to
/autopilot/locks/* and listeners for the generic dialog propagating changes
the other way around.
Set the target altitude when engaging alt hold.
Add a listener for /autopilot/locks/altitude (controlled from the generic
autopilot dialog).
Engage glideslope hold when the course selection knob is set to
"Localizer & glide path".
