client: Add a GTK example

[ids1024]

This provides an example of how to use wayland_client with a foreign display, in particular GTK. If there is a better way to use wayland_client with GTK, the example can be improved.

I'm not sure if this use of prepare_read is correct. I guess it can't really run into issues given no other thread is actually reading from the connection? Should it instead create a guard before calling unix_fd_add_local, and replace it with a new guard every time the function is called?

[secext2022]

This example no longer work.

After modify a little to compile, it run with crash:

thread 'main' panicked at tmp/src/main.rs:81:35:
called `Option::unwrap()` on a `None` value

panic at this line:

    glib::source::unix_fd_add_local(fd, glib::IOCondition::IN, move |_, _| {
        connection.prepare_read().unwrap().read().unwrap();  // !! PANIC !!
        ControlFlow::Continue
    });

[secext2022]

This is the new working version:

    glib::source::unix_fd_add_local(fd, glib::IOCondition::IN, move |_, _| {
        if let Some(g) = connection.prepare_read() {
            g.read().unwrap();
        } else {
            connection.backend().dispatch_inner_queue().unwrap();
        }
        ControlFlow::Continue
    });