wayland-client: add easier way to create object from wl_proxy pointer

[i509VCB]

Though this is possible via wayland-backend, users often do not immediately see how this can be done.

My proposal to make this work is an extension trait that is only implemented when a client_system feature is enabled. This would be easy to see in documentation.

pub trait WlProxyFromPtr: Proxy {
    unsafe fn from_ptr(conn: &Connection, ptr: *mut wl_proxy) -> Self;
}

#[cfg(feature = "client_system")]
impl<T: Proxy> WlProxyFromPtr for T {
    unsafe fn from_ptr(conn: &Connection, ptr: *mut wl_proxy) -> Self {
        let id = unsafe { ObjectId::from_ptr(Self::interface(), ptr) };
        Self::from_id(conn, id)
    }
}

[elinorbgr]

users often do not immediately see how this can be done

I wonder if this may be solved through documentation instead?

There is imo a strong appeal in having the *_system features contained to a single crate, as this removes a lot of surprises and headaches.