Autopilot: add interface properties and property-root.

Autopilot: add interface properties and property-root.

 - Support interface properties as with JSBSim for easy reuse
   and parametrization of autopilot components.
 - Add property-root property to allow changing property root
   for all relative paths. This allows easy use of multiple
   instances of the same autopilot component at the same time
   by specifiying different property root nodes.