atspi: Make EventBody generic over serializable types for kind

[DataTriny]

I still haven't figured out the best way forward for this, but at least now it's possible to serialize events holding a kind which type is something other than &str. At least I'll be able to take advantage of this for AccessKit.

• Added serde support to the State type.
• Rearranged EventBodyQT (I don't see why we would need a borrowing version of this type, it's kind of a bug in Qt so you wouldn't want anybody to use it on the server-side).
• Updated signals to use EventBody<&str> (except object's state_changed which now takes EventBody<State>).

Hopefully this can spark ideas on how to better type-check these event related structs.

[TTWNO]

Really great PR! Let me play around with it over the next few days so I understand what it's doing.

[DataTriny]

It's not doing much on client's side. You'd have to write a provider to take advantage of this. I'll ping you here once I've got something working with AccessKit so you can see a use case for it.

[DataTriny]

@TTWNO

You can find how it's used here.

To play with it locally:

git clone https://github.com/DataTriny/accesskit
cd accesskit/platforms/winit
git checkout atspi_basics
cargo run --release --example simple

You can see that Orca is able to detect focus state updates on the window and the buttons (we don't raise org.a11y.atspi.Event.Focus.Focus, only org.a11y.atspi.Event.Object.StateChange) signals.

[TTWNO]

This is really well done! Thank you, again, for your generous contributions to this library!