eframe app creation refactor

[emilk]

This refactors how eframe creates an app, removing the need for App::setup.

The purpose of this is to be able to access the following state before/during creation of ones app:

• egui::Context: alllow users to set fonts, style and maybe create textures.
• epi::Storage: for reading app state from storage.
• epi::Frame: can be cloned and stored so we can run request_repaint.
• glow::Context: for creating rendering resources.

Previously eframe was designed so that these were passed into App::setup, but that meant one would have to create an app before running the setup of it, making App::setup a weird "second constructor". This PR solves that problem.

App::name is now also removed, and is instead passed in by argument to eframe::run_native.

Additionally, eframe now re-exports everything in epi, so eframe users can ignore epi completely.

---

This now changes the eframe use to look like so:

EDIT: this has been edited to reflect the changes added in #1373

use eframe::egui;

#[cfg(not(target_arch = "wasm32"))]
fn main() {
    let native_options = eframe::NativeOptions::default();
    eframe::run_native("MyApp", native_options, Box::new(|cc| Box::new(MyEguiApp::new(cc))));
}

#[cfg(target_arch = "wasm32")]
#[wasm_bindgen]
pub fn start(canvas_id: &str) -> Result<(), eframe::wasm_bindgen::JsValue> {
    eframe::start_web(canvas_id, Box::new(|cc| Box::new(MyApp::new(cc))))
}

#[derive(Default)]
struct MyEguiApp {}

impl MyEguiApp {
    fn new(cc: &eframe::CreationContext<'_>) -> Self {
        // Customize egui here with cc.egui_ctx.set_fonts and cc.egui_ctx.set_visuals.
        // Restore app state using cc.storage (requires the "persistence" feature).
        // Use the cc.gl (a glow::Context) to create graphics shaders and buffers that you can use
        // for e.g. egui::PaintCallback.
        Self::default()
    }
}

impl eframe::App for MyEguiApp {
   fn update(&mut self, ctx: &egui::Context, frame: &eframe::Frame) {
       egui::CentralPanel::default().show(ctx, |ui| {
           ui.heading("Hello World!");
       });
   }
}

[DusterTheFirst]

This is a very welcome refactor! Thanks for the work you have put into this.

[AngelOnFira]

Sorry for the comment noise, just wanted to mention that it took me a while to find this while trying to understand why setup + name existed on ewebsocket's example app. This PR seems to give a good overview of how an app should be created; would it be worth specifying in some of eframe/epi's docs about how code used to look, so that people know how to migrate from setup if they need? If so, I can try upstreaming some docs.

I'm working on updating ewebsock's example app, so hopefully confusion shouldn't spawn from there in the future :)