Most subsystems are now created via the subsystem manager using the global
subsystem registrations.
The FGGlobals add_subsystem() and add_new_subsystem() methods have been removed
as the subsystem manager addition and creation function interface now exceeds
the functionality of these helper functions.
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.
