Replace eframe::Frame commands and WindowInfo with egui (#3564)

* Part of https://github.com/emilk/egui/issues/3556

## In short
You now almost never need to use `eframe::Frame` - instead use
`ui.input(|i| i.viewport())` for information about the current viewport
(native window), and use `ctx.send_viewport_cmd` to modify it.

## In detail

This PR removes most commands from `eframe::Frame`, and replaces them
with `ViewportCommand`.
So `frame.close()` becomes
`ctx.send_viewport_cmd(ViewportCommand::Close)`, etc.

`frame.info().window_info` is now also gone, replaced with `ui.input(|i|
i.viewport())`.

`frame.info().native_pixels_per_point` is replaced with `ui.input(|i|
i.raw.native_pixels_per_point)`.

`RawInput` now contains one `ViewportInfo` for each viewport.

Screenshots are taken with
`ctx.send_viewport_cmd(ViewportCommand::Screenshots)` and are returned
in `egui::Event` which you can check with:

``` ust
ui.input(|i| {
    for event in &i.raw.events {
        if let egui::Event::Screenshot { viewport_id, image } = event {
            // handle it here
        }
    }
});
```

### Motivation
You no longer need to pass around the `&eframe::Frame` everywhere.
This also opens the door for other integrations to use the same API of
`ViewportCommand`s.

examples/confirm_exit/src/main.rs

@@ -27,7 +27,7 @@ impl eframe::App for MyApp {
         self.allowed_to_close
     }
 
-    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
+    fn update(&mut self, ctx: &egui::Context, _frame: &mut eframe::Frame) {
         egui::CentralPanel::default().show(ctx, |ui| {
             ui.heading("Try to close the window");
         });
@@ -45,7 +45,7 @@ impl eframe::App for MyApp {
 
                         if ui.button("Yes!").clicked() {
                             self.allowed_to_close = true;
-                            frame.close();
+                            ui.ctx().send_viewport_cmd(egui::ViewportCommand::Close);
                         }
                     });
                 });