Make io related enums to support serde Serialize/Deserialize

[ruby3141]

Is your feature request related to a problem? Please describe.

Because imgui::Key does not support serde, this kind of thing is not possible.

use serde::{Deserialize, Serialize};
use imgui::Key;

#[derive(Serialize, Deserialize)]
pub struct HoykeyConfig {
    some_hotkey: Key,
    // ...
}

Providing proxy enum to use #[serde(remote="...")] is kind of viable, but not recommendable since imgui::Key has 140 entries.

Describe the solution you'd like

Make io related enums of this crate to derive serde::{Serialize, Deserialize}.