Fix window properties

[c0deaddict]

Getting the window_properties on nodes in the tree results in all None (on i3 version 4.20). This is caused by an unknown window_property "machine".

let mut i3 = I3Connection::connect().unwrap();
let tree = i3.get_tree().unwrap();
let node = find_focused(&tree).unwrap();
println!("{:?}", node.window_properties);

This PR fixes the issue of unknown window_properties resulting in an empty HashMap. And it also adds the "machine" window_property.

I also fixed some deprecation warnings (on rust 1.56.1).