Use tracing crate for logging

[emilk]

This adds the tracing crate to egui_web, egui_glow and egui-winit. egui gets it as an opt-in dependency. It is used for logging.

In egui_web the logs will automatically end up in the dev console. For native, app developers must add tracing-subscriber = "0.3" to their Cargo.toml and then add tracing_subscriber::fmt::init(); to their fn main in order to get logs to stdout. Even then, the egui crates log very little, so you may not see anything logged unless you run with RUST_LOG=debug.