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` ?

examples/custom_window_frame/src/main.rs

@@ -31,8 +31,8 @@ impl eframe::App for MyApp {
         egui::Rgba::TRANSPARENT.to_array() // Make sure we don't paint anything behind the rounded corners
     }
 
-    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
-        custom_window_frame(ctx, "egui with custom frame", |ui| {
+    fn ui(&mut self, ui: &mut egui::Ui, _frame: &mut eframe::Frame) {
+        custom_window_frame(ui.ctx(), "egui with custom frame", |ui| {
             ui.label("This is just the contents of the window.");
             ui.horizontal(|ui| {
                 ui.label("egui theme:");