Add a unit-test written by Huntely, and extend the AP configure code, so the
'time' config of monostables doesn't trigger an incorrect warning in the
console / logs.
Thanks to Huntley Palmer for pointing this out.
SF-ID: https://sourceforge.net/p/flightgear/codetickets/2812/
Allow making the AP source indirect (via a string property),
so that the actual source property can be configured or
adjusted from a -set.xml, or at runtime.
