Add helpers for zooming an app using Ctrl+Plus and Ctrl+Minus (#2239)

* Using tracing-subscriber in hello_world example

* Add Key::Plus/Minus/Equals

* Warn if failing to guess OS from User-Agent

* Remove jitter when using Context::set_pixels_per_point

* Demo app: zoom in/out using ⌘+ and ⌘-

* Demo app: make backend panel GUI scale slider better

* Optimize debug builds a bit

* typo

* Update changelog

* Add helper module `egui::gui_zoom` for zooming an app

* Better names, and update changelog

* Combine Plus and Equals keys

* Last fix

* Fix docs

examples/hello_world/Cargo.toml

@@ -10,3 +10,4 @@ publish = false
 
 [dependencies]
 eframe = { path = "../../crates/eframe" }
+tracing-subscriber = "0.3"

examples/hello_world/src/main.rs

@@ -3,6 +3,9 @@
 use eframe::egui;
 
 fn main() {
+    // Log to stdout (if you run with `RUST_LOG=debug`).
+    tracing_subscriber::fmt::init();
+
     let options = eframe::NativeOptions::default();
     eframe::run_native(
         "My egui App",