Seat identifiers on x11/wayland

Wayland is a multi-seat aware protocol, and I believe x11 is too. From this, I think it could be reasonable to add a platform specific extension trait unix::DeviceIdExt like so:

pub trait DeviceIdExt {
    /// Get the name of the seat this device is associated with
    ///
    /// Some systems can be multi-seat, meaning that there may be more than
    /// one keyboard / pointer, and these should be treated as different users interacting
    /// with the app.
    ///
    /// If this kind of multi-user interaction would be meaningful with your app, this
    /// method allows you to retrieve a seat-unique identifier from this `DeviceId`.
    fn seat(&self) -> &str;
}

While arguably pretty niche, this seat-awareness can be necessary for downstream apps. For example, on wayland, there is one clipboard per seat, and the app should be careful to trigger the copy and paste actions from the appropriate seat.

Thoughts on this design, @francesca64 ?

Start by tracing the DeviceId implementations and platform-specific X11 and Wayland integration in winit. Review how device identity is exposed and how multi-seat clipboard behavior affects downstream applications. Done would require an agreed API design and corresponding support for both platforms, but the issue does not define tests or a concrete implementation location.