FlightGear Merge Request #238: allow user to choose if aspect ratio should be autoadjusted with stereo split (merged)

This is my first merge request and contribution to flightgear, please correct me if I'm doing anything wrong.

In two words I want to be able to split the screen horizontally with a normal aspect ratio of the image, allowing me to use a VR headset mounted with an android phone.
All is explained here :
https://forum.flightgear.org/viewtopic.php?f=37&t=20055

To do this I created a new prop and allowed the stereo gui to handle this new property.

This merge request comes along with another in fgdata to handle the new prop.