Since https://github.com/emilk/egui/pull/1919 we can continue
the application after closing the native window. It therefore makes
more sense to call `frame.close()` to close the native window,
instead of `frame.quit()`.
* 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
