I love egui! Thank you Emil <3

This request specifically enables an `eframe::App` which stores a
lifetime.

In general, I believe this is necessary because `eframe::App` currently
does not seem to provide a good place to allocate and then borrow from
long-lived data between `update()` calls. To attempt to borrow such
long-lived data from a field of the `App` itself would be to create a
self-referential struct. A hacky alternative is to allocate long-lived
data with `Box::leak`, but that's a code smell and would cause problems
if a program ever creates multiple Apps.

As a more specific motivating example, I am developing with the
[inkwell](https://github.com/TheDan64/inkwell/) crate which requires
creating a `inkwell::context::Context` instance which is then borrowed
from by a bazillion things with a dedicated `'ctx` lifetime. I need such
a `inkwell::context::Context` for the duration of my `eframe::App` but I
can't store it as a field of the app. The most natural solution to me is
to simply to lift the inkwell context outside of the App and borrow from
it, but that currently fails because the AppCreator implicitly has a
`'static` lifetime requirement due to the use of `dyn` trait objects.

Here is a simpler, self-contained motivating example adapted from the
current [hello world example](https://docs.rs/eframe/latest/eframe/):

```rust
use eframe::egui;

struct LongLivedThing {
    message: String,
}

fn main() {
    let long_lived_thing = LongLivedThing {
        message: "Hello World!".to_string(),
    };

    let native_options = eframe::NativeOptions::default();
    eframe::run_native(
        "My egui App",
        native_options,
        Box::new(|cc| Ok(Box::new(MyEguiApp::new(cc, &long_lived_thing)))),
        //                                           ^^^^^^^^^^^^^^^^^
        //                                           BORROWING from long_lived_thing in App
    );
}

struct MyEguiApp<'a> {
    long_lived_thing: &'a LongLivedThing,
}

impl<'a> MyEguiApp<'a> {
    fn new(cc: &eframe::CreationContext<'_>, long_lived_thing: &'a LongLivedThing) -> 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.
        MyEguiApp { long_lived_thing }
    }
}

impl<'a> eframe::App for MyEguiApp<'a> {
    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
        egui::CentralPanel::default().show(ctx, |ui| {
            ui.heading(&self.long_lived_thing.message);
        });
    }
}
```

This currently fails to compile with:
```plaintext
error[E0597]: `long_lived_thing` does not live long enough
  --> src/main.rs:16:55
   |
16 |         Box::new(|cc| Ok(Box::new(MyEguiApp::new(cc, &long_lived_thing)))),
   |         ----------------------------------------------^^^^^^^^^^^^^^^^----
   |         |        |                                    |
   |         |        |                                    borrowed value does not live long enough
   |         |        value captured here
   |         cast requires that `long_lived_thing` is borrowed for `'static`
17 |     );
18 | }
   | - `long_lived_thing` dropped here while still borrowed
   |
   = note: due to object lifetime defaults, `Box<dyn for<'a, 'b> FnOnce(&'a CreationContext<'b>) -> Result<Box<dyn App>, Box<dyn std::error::Error + Send + Sync>>>` actually means `Box<(dyn for<'a, 'b> FnOnce(&'a CreationContext<'b>) -> Result<Box<dyn App>, Box<dyn std::error::Error + Send + Sync>> + 'static)>`
```

With the added lifetimes in this request, I'm able to compile and run
this as expected on Ubuntu + Wayland. I see the CI has been emailing me
about some build failures and I'll do what I can to address those.
Currently running the check.sh script as well.

This is intended to resolve emilk#2152

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* Closes emilk#2152
* [x] I have followed the instructions in the PR template