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/egui_kittest/src/builder.rs

@@ -167,6 +167,7 @@ impl<State> HarnessBuilder<State> {
     /// assert_eq!(*harness.state(), true);
     /// ```
     #[track_caller]
+    #[deprecated = "use `build_ui_state` instead"]
     pub fn build_state<'a>(
         self,
         app: impl FnMut(&egui::Context, &mut State) + 'a,
@@ -251,6 +252,7 @@ impl HarnessBuilder {
     /// ```
     #[must_use]
     #[track_caller]
+    #[deprecated = "use `build_ui` instead"]
     pub fn build<'a>(self, app: impl FnMut(&egui::Context) + 'a) -> Harness<'a> {
         Harness::from_builder(self, AppKind::Context(Box::new(app)), (), None)
     }