Pleased add timeout support

[brmmm3]

Pleased add Support for functions: add_timeout, repeat_timeout and remove_timeout.

[MoAlyousef]

Hi
I have added these methods in the app module. It's currently in the master branch. I've also added an example which mimics the example in the original FLTK docs:

use fltk::*;
use fltk::prelude::*;

fn callback() {
    println!("TICK");
    app::repeat_timeout(1.0, Box::new(callback));
}

fn main() {
    let app = app::App::default();
    let mut wind = window::Window::new(100, 100, 400, 300, "");
    wind.show();
    app::add_timeout(1.0, Box::new(callback));
    app.run().unwrap();
}

[brmmm3]

Many thanks for the very quick improvement!