eframe: Add a simplified native-only API for simple native apps

[emilk]

This lets you use eframe right from your main, with much less noise. This could be very useful for small, simple apps.

Example usage:

use eframe::egui;

fn main() -> Result<(), eframe::EframeError> {
    // Our application state:
    let mut name = "Arthur".to_owned();
    let mut age = 42;

    let options = eframe::NativeOptions::default();
    eframe::run_simple_native("My egui App", options, move |ctx, _frame| {
        egui::CentralPanel::default().show(ctx, |ui| {
            ui.heading("My egui Application");
            ui.horizontal(|ui| {
                let name_label = ui.label("Your name: ");
                ui.text_edit_singleline(&mut name)
                    .labelled_by(name_label.id);
            });
            ui.add(egui::Slider::new(&mut age, 0..=120).text("age"));
            if ui.button("Click each year").clicked() {
                age += 1;
            }
            ui.label(format!("Hello '{}', age {}", name, age));
        });
    })
}

It does the same as https://github.com/emilk/egui/blob/master/examples/hello_world/src/main.rs, but with much less code