Move examples out of eframe/examples into examples/ (#1486)

* Move examples out of eframe/examples into examples/

Give each example a `Cargo.toml` and `src/main.rs`.
This makes it easier for people to use as templates.

* Update README.md with more deps needed on vanilla Ubuntu
* Install libgtk-3-dev on CI, hoping that will fix something

examples/custom_3d_three-d/README.md

@@ -0,0 +1,16 @@
+This demo shows how to embed 3D rendering using [`three-d`](https://github.com/asny/three-d) in `eframe`.
+
+Any 3D library built on top of [`glow`](https://github.com/grovesNL/glow) can be used in `eframe`.
+
+Alternatively you can render 3D stuff to a texture and display it using [`egui::Ui::image`].
+
+If you are content of having egui sit on top of a 3D background, take a look at:
+
+* [`bevy_egui`](https://github.com/mvlabat/bevy_egui)
+* [`three-d`](https://github.com/asny/three-d)
+
+
+
+```sh
+cargo run -p custom_3d_three-d
+```