Introduce boarding stairs with animations and configs

[nico-castell]

In Models/Ground/Services/Stairs there was a 3D model for boarding stairs and a few liveries. But there was no menu to enable them.

I've created a subsection of the Ground Services dialog, called Boarding Services.

In this subsection there's a checkbox to enable the boarding stairs truck, and two buttons, one to drive said truck towards and away from the door. And another to access a configuration menu.

This configuration menu allows people to choose one of the liveries in Models/Ground/Services/Stairs for their stairs, and to enable/disable the cover (maybe it's raining).

Here's a demonstration:

[Marsdolphin]

Reviewing this in sim for verification

[Marsdolphin]

Merging