Allow to find descendents at a certain depth.

[simesy]

Currently findDescendents() searches down to a bottom depth, and I'd also like to be able to pass a top depth.

This is not critical functionality for me, but I think it would be a nice feature. I'm about to do a patch on the drupal module that would use this feature if it was available.

My proposal i don't think would break any existing functionality. But I could also create a new method findDescendentsAtDepth

  public function findDescendants(NodeKey $nodeKey, $depth = 0) {

becomes

  public function findDescendants(NodeKey $nodeKey, $end_depth = 0, $start_depth = 0) {

[simesy]

The order of arguments is a bit weird, but you're much more likely to set end depth, so it makes sense.