* Part of #5113
* Part of #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)
  - #7779

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

* Part of emilk#3524

This is a breaking change, as it changes the how embedded viewports
work.
Before it was up to the user to display a `egui::Window` if they wanted.
Now egui creates an `egui::Window` for you, so you only need to add the
contents.

To signal this change in behavior, `ViewportClass::Embedded` is gone and
is now called `ViewportClass::EmbeddedWindow`.

* Part of emilk#5113
* Part of emilk#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)
  - emilk#7779

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