Apparently it's not only replaced elements that can have intrinsic
sizes, so let's move this concept from ReplacedBox to Box. To avoid
bloating Box, we make the accessors virtual.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
