Use glib's native async support for handling events instead of a sync glib channel

[mmstick]

Basically

let (sender, mut receiver) = futures::channel::mpsc::unbounded();

// Create widgets and connect signals to clones of the sender

glib::MainContext::default().spawn_local(async move {
    // Capture widgets and application state in this main async function
    while let Some(event) = receiver.next().await {
        match event {}
    }
});

For the GTK application, since it already spawns an async runtime for handling GTK signals