Merge all backend crates into egui_backend with feature gates.

[Red Artist]

the current method of having separate crates for all the backends is leading to high friction when we are working.

1. making examples requires us to add ALL backends to the dev-dependencies and makes ALL crates compile.
2. by feature gating them in the egui_backend crate itself, we keep all the implementations in the same place.
3. also, publishing is easier

[Red Artist]

fixed with a1ba7cf