egui demo library

This crate contains example code for egui.

The demo library is a separate crate for three reasons:

• To ensure it only uses the public egui api.
• To remove the amount of code in egui proper.
• To make it easy for 3rd party egui integrations to use it for tests.
  • See for instance https://github.com/not-fl3/egui-miniquad/blob/master/examples/demo.rs