Add gtk-egui-area integration to README (#5129)

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* The PR title is what ends up in the changelog, so make it descriptive!
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to test and add commits to your PR.
* Remember to run `cargo fmt` and `cargo clippy`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->
* [x] I have followed the instructions in the PR template

Hi, I've made an integration that allows egui to be embedded as a widget
inside of a GTK application. I'm planning to use it to draw interactive
plots via `egui_plot`, as I haven't found comparable plotting widgets in
GTK. I hope others will find this integration useful as well.

The library doesn't do its own rendering, it instead relies on
`egui_glow` to render inside of a GTK-provided OpenGL drawing area.

It is published on [crates.io](https://crates.io/crates/gtk-egui-area).

README.md

@@ -204,6 +204,7 @@ These are the official egui integrations:
 * [`fltk-egui`](https://crates.io/crates/fltk-egui) for [fltk-rs](https://github.com/fltk-rs/fltk-rs)
 * [`ggegui`](https://github.com/NemuiSen/ggegui) for the [ggez](https://ggez.rs/) game framework
 * [`godot-egui`](https://github.com/setzer22/godot-egui) for [godot-rust](https://github.com/godot-rust/godot-rust)
+* [`gtk-egui-area`](https://github.com/ilya-zlobintsev/gtk-egui-area) for [gtk-rs](https://github.com/gtk-rs/gtk4-rs)
 * [`nannou_egui`](https://github.com/nannou-org/nannou/tree/master/nannou_egui) for [nannou](https://nannou.cc)
 * [`notan_egui`](https://github.com/Nazariglez/notan/tree/main/crates/notan_egui) for [notan](https://github.com/Nazariglez/notan)
 * [`screen-13-egui`](https://github.com/attackgoat/screen-13/tree/master/contrib/screen-13-egui) for [Screen 13](https://github.com/attackgoat/screen-13)