Hold option for AI aircraft on ground.

[Iceviper]

It would be very useful if we could hold ai aircraft when taxiing. Would allow for a mission scripted atc with custom voice overs. So, for example, we could hold an aircraft at the active runway while it is given clearance to depart or to hold aircraft in zones or on ramps while others taxi.

[MAXsenna]

Hopefully this will be better with the new ATC.