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 *
This also adds an API to Label to determine if the Label itself or its
child TextNode is hovered. This allows ButtonBox to render in a hovered
state when the label is hovered.
