Add ability to use Ailevator function to match elevator servos

It would be great if there was the ability to independently set up / down travels for servo 1 and 2. Currently, up / down are set independently, but not for each servo. As an example, this would allow quick servo matching at low, medium and high rate.

A similar result can be achieved using the servo balancer – but much simpler to do here….. And the servo balancer does not work so well….. (if you have subtrim, you can't seem to move the center point - so your neutral value changes when you balance?)