A Shortcut can now be either have a keyboard key, or a mouse button,
along with any modifiers.
Decided to add an extra type field instead of subclassing, which means
callers will have to be a little careful before accessing a particular
input method's "key", but it keeps things simple for now.
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 *
