iced layout markup language & hotswap test

This was created for iced-rs/iced#21

cargo run

todo:

• Templated layout engine
  • ron-based markup language
  • Templates handled with handlebars
• Handle user-defined messages for callbacks
  • User-defined message type must be serde::Deserialize
• User-defined state
• Hot-reloading
  • Missing a Subscription that watches for file changes
• Implement all iced_native widgets
• Custom widgets?
• fix std::mem::transmute horrors