Support for numpad

[dancavolix]

Hi. First of all, congrats on the project. Amazing stuff. It does what it proposes and is also very easy for beginners like me to learn how to use. I'm currently using it on a project of mine.

So, I've noticed a lack of support for the numerical keys on the numpad. On further inspection, this happens because they aren't listed in the Key enum in src/keyboard.rs

pub enum Key {
    // --snip--
    Num0,
    Num1,
    Num2,
    // --snip--

and below, on the same file,

fn transform_device_key(device_key: &dq::Keycode) -> Key {
        match device_key {
            // --snip--
            dq::Keycode::Key0 => Key::Num0,
            dq::Keycode::Key1 => Key::Num1,
            dq::Keycode::Key2 => Key::Num2,
            // --snip--

So in order to add numpad support, it is just a matter of listing the NumpadX key enumeration and later, inside the function, add dq::Keycode::NumpadX => Key::NumpadX for each key. Since these changes were so simple, I've already implemented them in this fork. I've also added them as motion key alternatives in the space invaders example.

[lemunozm]

Hi!

Thanks for adding them! It would be awesome to have these changes in the main repository. Do you want to open a PR with it?

[lemunozm]

Done in #45