Steps through an "array" of property settings. The first argument
specifies a destination property. The second is a string containing
a global property tree. This tree should contain an array of
indexed <setting> children. This function will maintain a
<current-setting> child, which contains the index of the currently
active setting. The third argument specifies an integer delta,
indicating how many steps to move through the setting array.
Note that because of the magic of the property system, this
mechanism works for all scalar property types (bool, int, double,
string).
TODO: This interface could easily be extended to allow for wrapping,
in addition to clamping, allowing a "cycle" of settings to be
defined. It could also be hooked up with the interpolate() call,
which would allow the removal of the transition-time feature from
YASim. Finally, other pre-existing features (the views and engine
magnetos, for instance), work similarly but not compatibly, and
could be integrated.