[css-multicol-1][css-display-3] Introduce a 'display' value for multi-column layout #1352
Description
To have control over the multi-column display defined by the column-* properties in the CSS Multi-column Layout 1, a columns value should be added to the <display-inside> value defined in CSS Display 3.
Making column layout another display type solves the explicitly noted exclusion in other layout specs like CSS Grid Layout 1 or CSS Flexible Box Layout 1 saying
the column-* properties in the Multi-column Layout module have no effect on a * container.
in a nice way.
Furthermore, it would allow to merge the different *-gap properties to an item-gap (suggested in the discussion about adding a flex-items gap property).
Sebastian