Implement \once \revert

Implement \once \revert


Contains commits:

Document \once \revert


Regtest for \once \revert


Let \displayLilyMusic heed \once \unset and \once \revert


Implement \once \revert in Pop_property_iterator


Add Grob_property_info::value_when_pop function


Factor Grob_property_info::update_alist out of Grob_property_info::updated


Let Push_property_iterator only revert when \once\override succeeded