The autopilot has grown beyond being just an autopilot. It's
components, filters and controllers could be used to set property
values based on complex rules and driven from other property values.
That's why I chose to give it the name "property-rule".
This patch allows to define an arbitrary number of property rules under
a/sim/property-rule[n]/path
using the same syntax as autopilot configurations.
Split the former single-file-implementation in xmlauto.?xx into multiple
files and use some OO techniques. Started with documentation to be used
with doxygen.
