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 *
Most of the functions under FileUtils were removed, except those which
dealt with file deletion, as they spawned MessageBoxes for errors, as
such, those functions were written in terms of Core::File::remove.