eframe: Replace Frame::update with fn logic and fn ui (#7775)

* Part of https://github.com/emilk/egui/issues/5113
* Part of https://github.com/emilk/egui/issues/3524

## What
This deprecates `eframe::App::update` and replaces it with two new
functions:

```rs
pub trait App {
	/// Called just before `ui`, and in the future this will
    /// also be called for background apps when needed.
	fn logic(&mut self, ctx: &egui::Context, frame: &mut Frame) { }
	
    /// Show your user interface to the user.
	fn ui(&mut self, ui: &mut egui::Ui, frame: &mut Frame);

	…
}
```

Similarly, `Context::run` is deprecated in favor of `Context::run_ui`.

`Plugin`s are now handed a `Ui` instead of just a `Context` in
`on_begin/end_frame`.

## TODO
…either in this PR or a later one
* [x] Deprecate `App::update`
* [x] Deprecate `Context::run`
* [x] Change plugins to get a `Ui`
* [x] Update kittest
* [x] Change viewports to get UI:s (`show_viewport_immediate` etc)
  - https://github.com/emilk/egui/pull/7779

## Later PRs
* [ ] Deprecate `Panel::show`
* [ ] Deprecate `CentralPanel::show`
* [ ] Deprecate `CentralPanel` ?

crates/eframe/src/native/wgpu_integration.rs

@@ -1027,8 +1027,8 @@ fn render_immediate_viewport(
         shapes,
         pixels_per_point,
         viewport_output,
-    } = egui_ctx.run(input, |ctx| {
-        viewport_ui_cb(ctx);
+    } = egui_ctx.run_ui(input, |ui| {
+        viewport_ui_cb(ui);
     });
 
     // ------------------------------------------
@@ -1155,7 +1155,7 @@ fn initialize_or_update_viewport<'a>(
     ids: ViewportIdPair,
     class: ViewportClass,
     mut builder: ViewportBuilder,
-    viewport_ui_cb: Option<Arc<dyn Fn(&egui::Context) + Send + Sync>>,
+    viewport_ui_cb: Option<Arc<dyn Fn(&mut egui::Ui) + Send + Sync>>,
     painter: &mut egui_wgpu::winit::Painter,
 ) -> &'a mut Viewport {
     use std::collections::btree_map::Entry;