[ISSUE] Implement \once \revert

[David Kastrup]

Status: Started
Type: Enhancement
Patch: New
Rietveld issue: 265090043 (https://codereview.appspot.com/265090043)
Description:

Implement \once \revert

This is actually cheating in that it instead does a \once \override with
the value that would result from reverting, somewhat similar to how
\once \unset works.  The results may correspond to a different reality
when occuring in recorded music (music quotes, \partcombine and
similar) but will at least have a predictable net effect on the property
stack.

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



-- 
David Kastrup