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 *
Commit 6a6f19a72 broke the cell position display in the top left of the
Spreadsheet window and the title of the cell type dialog, causing the
application to crash when interacting with cells beyond column FE.
This will make constructing (and destructing) Positions a lot cheaper
(as it no longer needs to ref() and unref() a String).
Resulted from #5483, but doesn't fix it.
