Add a CC0-1.0 license option for the notify-types crate

[musicinmybrain]

This changes the license of the notify-types crate from

MIT OR Apache-2.0

to

MIT OR Apache-2.0 OR CC0-1.0

and adds a copy of LICENSE-CC0 from the main notify crate, as requested in #661 (comment).

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I don’t have a personal interest in this change – in fact, it makes my life very slightly more difficult as a downstream package maintainer, since CC0-1.0 is not-allowed for code in Fedora and I would need to patch it out of the choices in the license expression. Still, this PR ties up the last loose end from the discussion in #661.

[dfaust]

in fact, it makes my life very slightly more difficult as a downstream package maintainer, since CC0-1.0 is not-allowed for code in Fedora

In that case, let's not do it.
But what about notify? That crate is still licensed under CC0-1.0.

[musicinmybrain]

In that case, let's not do it. But what about notify? That crate is still licensed under CC0-1.0.

We have the following policy:

Existing uses of CC0-1.0 on code files in Fedora packages prior to 2022-08-01, and subsequent upstream versions of those files in those packages, continue to be allowed. We encourage Fedora package maintainers to ask upstreams to relicense such files.

This exception covers our rust-notify package, which we have had since 2019-03-13, and I believe it covers notify-types/src/event.rs prior to #662 since it was not a new CC0-1.0-licensed source file, but was simply moved or split out from the notify crate.

We would not be able to package the notify crate at all if we hadn’t done so prior to 2022-08-01.

If a relicensing of the remaining CC0-1.0 code is possible (presumably with the consent of all relevant contributors), then that would certainly be welcome.

[0xpr03]

There is an open issue, I fear notify is stuck in re licensing limbo hell.