It is possible to propagate a notification down the Node tree by
using `propagate_notification`, but there was no such method for
doing the same but with method calls.
This commit adds the `propagate_call` method, which calls a method
on a node and all child nodes. An optional paramter `parent_first`
determines whether the parent node gets called before or after the
children have been visited. It defaults to false, so the parent
gets called last.
390f7def39