X11: Implement putting things into the clipboard

[psychon]

This PR fixes #937: The X11 platform now fully supports the clipboard.

When putting things into the clipboard, a new (invisible (= not mapped)) window is created and this window becomes the selection owner. Nothing else "x11-y" happens, so the clipboard contents are just kept in memory. Later, we might get SelectionRequest events, which are replied to by setting a window property and sending an event. INCR requests (for "incremental" (for "lots of data")) are again more complicated, just as when getting the clipboard contents.

Another complication are self-transfer: If we own the CLIPBOARD selection and request the clipboard contents, we would hang: The X11 server sends us an event that something wants a selection transfer, but the code would just put the event into a queue and not answer it. I briefly tried to implement "we still have to handle these requests", but ran into borrowing problems (the code for getting the selection and handling selection requests had conflicting borrows). Since it is way easier this way anyway, I implement a short-circuit: When we are the selection owner, "nothing X11" is done and we just directly provide the requested data.

Testing done with the following program and pasting into gvim (I also tested without the while loop for non-INCR transfers):

fn main() {
    tracing_subscriber::fmt().init();
    let app = druid_shell::Application::new().unwrap();
    let mut contents = String::from("1234567890");
    while contents.len() < 17 * 1024 * 1024 {
        let copy = contents.clone();
        contents = contents + &copy;
    }
    app.clipboard().put_string(contents);
    println!("{:?}", app.clipboard().available_type_names());
    println!("{:?}", app.clipboard().preferred_format(&["FOOBAR", "UTF8_STRING"]));
    println!("{:?}", app.clipboard().get_string().map(|s| s.len()));
    println!("Running!");
    app.run(None);
}

[maan2003]

Thanks! Really happy to see x11 backend moving forward. Just some minor nits:

[maan2003]

Thanks!