org-depend TRIGGER chain-siblings(NEXT) property is hardly usable for
me, because it requires too much effort to keep items nicely sorted.

For example if next headline is already in DONE state, chain-siblings
would still change it. I prefer to sort my items by setting their
priorities and/or effort estimate, leaving DONE items in place for
some time.

Attached patch implements new TRIGGER chain-find-next(NEXT[,options])
trigger, which allows to flexibly select which of the siblings will be
changed to NEXT.

Example: chain-find-next(NEXT,from-current,priority-up,todo-only)