Backport layout block support layout definition and stabilisation changes for 6.3#4624

andrewserong wants to merge 7 commits intoWordPress:trunkWordPress/wordpress-develop:trunkfrom

andrewserong:update/move-layout-definitions-to-a-functionandrewserong/wordpress-develop:update/move-layout-definitions-to-a-functionCopy head branch name to clipboard