selector: Rework iterating over subnodes

selector: Rework iterating over subnodes

Instead of a foreach() function, introduce an iterator, so that the
caller can drive the iteration.

This allows doing stuff inbetween callbacks and avoids closures when
more than one data object should be passed.

As a side effect I even get a small, but noticeable performance
improvement in the 2-10% range depending on benchmark, I guess that's
because there's no function pointer passing going on anymore.