Add resizable widget

[jrmoulton]

Creating a resizable widget is now as simple as creating a regular stack.

let view = resizable::resizable((side_bar, main_window))
    .style(|s| s.width_full().height_full())
    .custom_style(move |s| {
        s.handle_color(dragger_color)
            .active(|s| s.handle_color(active_dragger_color))
    });