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_keypad/src/main.rs

@@ -44,11 +44,11 @@ impl Default for MyApp {
 }
 
 impl eframe::App for MyApp {
-    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
+    fn ui(&mut self, ui: &mut egui::Ui, _frame: &mut eframe::Frame) {
         egui::Window::new("Custom Keypad")
             .default_pos([100.0, 100.0])
             .title_bar(true)
-            .show(ctx, |ui| {
+            .show(ui.ctx(), |ui| {
                 ui.horizontal(|ui| {
                     ui.label("Your name: ");
                     ui.text_edit_singleline(&mut self.name);
@@ -60,7 +60,7 @@ impl eframe::App for MyApp {
                 ui.label(format!("Hello '{}', age {}", self.name, self.age));
             });
 
-        self.keypad.show(ctx);
+        self.keypad.show(ui.ctx());
     }
 
     fn raw_input_hook(&mut self, ctx: &egui::Context, raw_input: &mut egui::RawInput) {