Mark Space widget as Copy and Clone

[Shadowcat650]

This makes usage more ergonomic when multiple Space identical objects are used.

For example, this situation:

let search_header = row![
    Space::new().width(Length::FillPortion(1)),
    searchbar.width(Length::FillPortion(4)),
    Space::new().width(Length::FillPortion(1)),
];

Becomes:

let space = Space::new().width(Length::FillPortion(1));
let search_header = row![
    space,
    searchbar.width(Length::FillPortion(4)),
    space,
];