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/user_attention/src/main.rs

@@ -1,14 +1,12 @@
-use eframe::{
-    egui::{Button, CentralPanel, Context, UserAttentionType},
-    CreationContext, NativeOptions,
-};
+use eframe::{egui, CreationContext, NativeOptions};
+use egui::{Button, CentralPanel, Context, UserAttentionType};
 
 use std::time::{Duration, SystemTime};
 
 fn main() -> eframe::Result<()> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let native_options = NativeOptions {
-        initial_window_size: Some(eframe::egui::vec2(400., 200.)),
+        initial_window_size: Some(egui::vec2(400., 200.)),
         ..Default::default()
     };
     eframe::run_native(
@@ -54,11 +52,11 @@ impl Application {
 }
 
 impl eframe::App for Application {
-    fn update(&mut self, ctx: &Context, frame: &mut eframe::Frame) {
+    fn update(&mut self, ctx: &Context, _frame: &mut eframe::Frame) {
         if let Some(request_at) = self.request_at {
             if request_at < SystemTime::now() {
                 self.request_at = None;
-                frame.request_user_attention(self.attention);
+                ctx.send_viewport_cmd(egui::ViewportCommand::RequestUserAttention(self.attention));
                 if self.auto_reset {
                     self.auto_reset = false;
                     self.reset_at = Some(SystemTime::now() + Self::attention_reset_timeout());
@@ -69,7 +67,9 @@ impl eframe::App for Application {
         if let Some(reset_at) = self.reset_at {
             if reset_at < SystemTime::now() {
                 self.reset_at = None;
-                frame.request_user_attention(UserAttentionType::Reset);
+                ctx.send_viewport_cmd(egui::ViewportCommand::RequestUserAttention(
+                    UserAttentionType::Reset,
+                ));
             }
         }
 
@@ -77,7 +77,7 @@ impl eframe::App for Application {
             ui.vertical(|ui| {
                 ui.horizontal(|ui| {
                     ui.label("Attention type:");
-                    eframe::egui::ComboBox::new("attention", "")
+                    egui::ComboBox::new("attention", "")
                         .selected_text(repr(self.attention))
                         .show_ui(ui, |ui| {
                             for kind in [