Issue 4609/4: Move \once action from iterators to listeners

Issue 4609/4: Move \once action from iterators to listeners

This ends the dependency of the events generated for \once\unset and
\once\set on the current context (bad for recording and replaying events
like with the part combiner and quoted music).  It also implements
\once\revert and makes every \once\override and \once\revert impervious
to any other overrides and reverts that may happen at the same time.