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 *
When a button has a menu, it opens the menu on mousedown and the
menu gains input focus immediately. While the menu is open, the
button is painted in a pressed state.
